* {
  margin: 0;
  padding: 0;
  box-sizing: border-box
}
/* Prevent pull-to-refresh on iOS/Android */
html, body {
  overscroll-behavior-y: none;
}

/* ===== THEME SYSTEM ===== */
:root {
  --font: system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
  --sans: var(--font);
  --mono: ui-monospace, "SF Mono", "Cascadia Mono", monospace;
  --bw: 2px;
  --rad: 10px;
  --rad-lg: 16px;

  /* Easing curves */
  --ease-out: cubic-bezier(0.16, 1, 0.3, 1);
  --ease-in-out: cubic-bezier(0.4, 0, 0.2, 1);
  --ease-spring: cubic-bezier(0.34, 1.56, 0.64, 1);

  /* Touch feedback */
  --active-scale: 0.97;

  /* Timing */
  --dur-fast: 150ms;
  --dur: 250ms;
  --dur-slow: 400ms;

  /* Unified shadow system */
  --shadow-xs: 0 1px 2px rgba(0,0,0,.06);
  --shadow-sm: 0 2px 6px rgba(0,0,0,.08);
  --shadow: 0 4px 16px rgba(0,0,0,.1), 0 1px 3px rgba(0,0,0,.06);
  --shadow-lg: 0 8px 30px rgba(0,0,0,.12), 0 2px 8px rgba(0,0,0,.06);
  --shadow-xl: 0 16px 50px rgba(0,0,0,.16), 0 4px 12px rgba(0,0,0,.08);
}

[data-theme="dark"] {
  --bg: #0a0a0a;
  --bg2: #131316;
  --bg3: #1c1c20;
  --bg4: #2a2a2e;
  --bg5: #38383d;
  --tx: #f2f2f7;
  --tx1: #d1d1d6;
  --tx2: #98989f;
  --tx3: #636366;
  --bdr: #2c2c30;
  --bdr2: #3a3a3e;
  --ac: #c8ff00;
  --ac2: #e2ff66;
  --ac-tx: #0a0a0a;
  --ac-sh: var(--shadow-sm);
  --ac-sh-sm: var(--shadow-xs);
  --gn: #30d158;
  --gn-bg: rgba(48, 209, 88, .1);
  --gn-bdr: #30d158;
  --rd: #ff453a;
  --rd-bg: rgba(255, 69, 58, .1);
  --rd-bdr: #ff453a;
  --yl: #ffd60a;
  --cy: #64d2ff;
  --or: #ff9f0a;
  --card-sh: var(--shadow);
  --card-sh-sm: var(--shadow-sm);
  --hover-bg: rgba(255,255,255,.04);
}

[data-theme="light"] {
  --bg: #ffffff;
  --bg2: #ffffff;
  --bg3: #f7f7f8;
  --bg4: #e8e8ec;
  --bg5: #d1d1d6;
  --tx: #1a1a1a;
  --tx1: #333;
  --tx2: #555;
  --tx3: #777;
  --bdr: #d4d4d8;
  --bdr2: #a1a1aa;
  --ac: #4400ff;
  --ac2: #6633ff;
  --ac-tx: #fff;
  --ac-sh: var(--shadow) #220088;
  --ac-sh-sm: var(--shadow-sm) #220088;
  --gn: #00994d;
  --gn-bg: rgba(0, 153, 77, .08);
  --gn-bdr: #00994d;
  --rd: #cc0000;
  --rd-bg: rgba(204, 0, 0, .06);
  --rd-bdr: #cc0000;
  --yl: #cc8800;
  --cy: #0077aa;
  --or: #cc6600;
  --card-sh: var(--shadow);
  --card-sh-sm: var(--shadow-sm);
  --hover-bg: rgba(0,0,0,.04);
}

html {
  height: 100%;
  scroll-behavior: smooth;
}

body {
  font-family: var(--font);
  background: var(--bg);
  color: var(--tx);
  line-height: 1.65;
  height: 100%;
  margin: 0;
  padding: 0;
  overflow: hidden;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-rendering: optimizeLegibility;
  transition: background var(--dur) var(--ease-in-out), color var(--dur) var(--ease-in-out)
}

button {
  cursor: pointer;
  font-family: var(--font);
  border: none;
  border-radius: var(--rad);
  transition: transform var(--dur-fast) var(--ease-out), box-shadow var(--dur) var(--ease-out), background var(--dur-fast), opacity var(--dur-fast)
}

button:focus-visible {
  outline: 3px solid var(--ac);
  outline-offset: 2px
}

input,
textarea {
  font-family: var(--mono);
  border-radius: var(--rad);
  border: var(--bw) solid var(--bdr);
  background: var(--bg);
  color: var(--tx);
  padding: 10px 14px;
  font-size: 14px;
  transition: border-color var(--dur) var(--ease-out), box-shadow var(--dur) var(--ease-out)
}

input:focus,
textarea:focus {
  border-color: var(--ac);
  box-shadow: 0 0 0 3px rgba(200, 255, 0, .15)
}

input:focus,
textarea:focus {
  outline: none;
  border-color: var(--ac)
}

::-webkit-scrollbar {
  width: 8px
}

::-webkit-scrollbar-track {
  background: var(--bg)
}

::-webkit-scrollbar-thumb {
  background: var(--bdr);
  border: 2px solid var(--bg)
}

/* ===== LAYOUT ===== */
#app {
  display: flex;
  height: 100%
}

/* Sidebar */
#sidebar {
  width: 300px;
  min-width: 300px;
  background: var(--bg2);
  border-right: var(--bw) solid var(--bdr);
  display: flex;
  flex-direction: column;
  transition: transform var(--dur-slow) var(--ease-out), width var(--dur-slow) var(--ease-out), min-width var(--dur-slow) var(--ease-out)
}

#sidebar.collapsed {
  transform: translateX(-100%);
  width: 0;
  min-width: 0;
  overflow: hidden
}

.sb-head {
  padding: 14px 16px;
  border-bottom: var(--bw) solid var(--bdr);
  display: flex;
  align-items: center;
  gap: 12px
}

.sb-logo {
  font-family: var(--mono);
  font-size: 32px;
  font-weight: 700;
  color: var(--ac);
  line-height: 1
}

.sb-head h1 {
  font-size: 16px;
  font-weight: 700;
  letter-spacing: -0.3px
}

.sb-progress {
  margin: 6px 10px 4px;
  padding: 6px 10px;
  background: var(--bg3);
  border: 2px solid var(--bdr);
  display: flex;
  align-items: center;
  gap: 8px
}

.pbar {
  flex: 1;
  height: 10px;
  background: var(--bg);
  border: 2px solid var(--bdr)
}

.pfill {
  height: 100%;
  transition: width .4s
}

.pfill-main {
  background: var(--ac)
}

.pct-label {
  font-family: var(--mono);
  font-size: 12px;
  font-weight: 700;
  color: var(--ac);
  min-width: 36px;
  text-align: right
}

#map {
  flex: 1;
  overflow-y: auto;
  padding: 8px 10px
}

.mi {
  margin-bottom: 4px
}

.mh {
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 10px 12px;
  cursor: pointer;
  font-size: 12px;
  font-weight: 700;
  color: var(--tx2);
  background: none;
  width: 100%;
  text-align: left;
  letter-spacing: 0.3px;
  border: 2px solid transparent;
  transition: all var(--dur-fast) var(--ease-out)
}

.mh:hover {
  color: var(--tx);
  background: var(--hover-bg);
  border-color: var(--bdr)
}

.mh.exp {
  color: var(--tx)
}

.mh .arr {
  font-size: 8px;
  transition: transform .2s;
  width: 12px;
  flex-shrink: 0;
  font-family: var(--mono)
}

.mh.exp .arr {
  transform: rotate(90deg)
}

.mt {
  flex: 1
}

.mp {
  width: 36px;
  height: 6px;
  background: var(--bg);
  border: 1px solid var(--bdr);
  flex-shrink: 0
}

.mpf {
  height: 100%;
  background: var(--gn)
}

.ll {
  overflow: hidden;
  max-height: 0;
  transition: max-height var(--dur-slow) var(--ease-out)
}

.ll.exp {
  max-height: 600px
}

.li {
  padding: 8px 12px 8px 38px;
  font-size: 12px;
  color: var(--tx3);
  cursor: pointer;
  display: flex;
  align-items: center;
  gap: 8px;
  border-left: 3px solid transparent;
  margin-left: 12px;
  transition: all .12s
}

.li:hover {
  color: var(--tx);
  background: var(--hover-bg);
  border-left-color: var(--ac)
}

.li.act {
  color: var(--ac-tx);
  background: var(--ac);
  border-left-color: var(--ac);
  font-weight: 700
}

.sb-foot {
  padding: 6px;
  border-top: var(--bw) solid var(--bdr);
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 3px
}

.sb-foot button {
  padding: 6px 4px;
  font-size: 9px;
  font-weight: 700;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 3px;
  white-space: nowrap;
  letter-spacing: 0.2px;
  background: var(--bg3);
  color: var(--tx2);
  border: 1px solid var(--bdr);
  transition: all var(--dur-fast) var(--ease-out)
}

.sb-foot button:hover {
  background: var(--ac);
  color: var(--ac-tx);
  border-color: var(--ac)
}

/* Main */
#main {
  flex: 1;
  display: flex;
  flex-direction: column;
  overflow: hidden
}

#topbar {
  display: flex;
  align-items: center;
  gap: 12px;
  padding: 12px 24px;
  padding-left: 80px;
  /* Push right to clear macOS window controls */
  border-bottom: var(--bw) solid var(--bdr);
  background: var(--bg2)
}

.tb-btn {
  background: none;
  color: var(--tx2);
  font-size: 18px;
  padding: 6px 10px;
  border: 2px solid transparent;
  transition: all .15s
}

.tb-btn:hover {
  color: var(--ac);
  border-color: var(--ac)
}

#tbtitle {
  font-size: 14px;
  font-weight: 600;
  flex: 1;
  letter-spacing: -0.2px
}

.theme-toggle {
  background: var(--bg3);
  color: var(--tx);
  border: 2px solid var(--bdr);
  padding: 6px 12px;
  font-size: 14px;
  font-weight: 700;
  transition: all .15s;
  cursor: pointer
}

.theme-toggle:hover {
  border-color: var(--ac);
  color: var(--ac)
}

#wrap {
  flex: 1;
  overflow-y: auto;
  -webkit-overflow-scrolling: touch;
  overscroll-behavior-y: none;
}

#ct {
  padding: 40px;
  max-width: 880px;
  margin: 0 auto
}

/* ===== TYPOGRAPHY ===== */
.ls {
  margin-bottom: 36px
}

.ls h2 {
  font-size: 28px;
  font-weight: 700;
  margin-bottom: 16px;
  letter-spacing: -1px;
  line-height: 1.2
}

.ls h3 {
  font-size: 13px;
  font-weight: 700;
  margin-bottom: 10px;
  color: var(--ac);
  letter-spacing: 0.3px
}

.ls p,
.cb p,
.eb p {
  font-size: 14px;
  color: var(--tx2);
  margin-bottom: 10px;
  line-height: 1.7
}

/* Concept box */
.cb {
  background: var(--bg3);
  border: var(--bw) solid var(--bdr);
  padding: 20px 24px;
  margin: 20px 0;
  box-shadow: var(--card-sh);
  position: relative
}

.cb::before {
  content: "";
  position: absolute;
  top: -1px;
  left: -1px;
  width: 6px;
  height: calc(100% + 2px);
  background: var(--ac)
}

.cb h4 {
  font-size: 11px;
  font-weight: 700;
  color: var(--ac);
  text-transform: uppercase;
  letter-spacing: 0.8px;
  margin-bottom: 10px
}

/* List markers (bullets, numbers) inside .cb / .eb / .ls need padding-left
   so the marker doesn't render on top of the orange callout border or
   off the visible content area on narrow viewports. With list-style-
   position: outside (the default), the marker sits ~1em LEFT of the list's
   own box, so we need at least 24px of left padding to clear the border. */
.cb ul, .cb ol,
.eb ul, .eb ol,
.ls ul, .ls ol {
  padding-left: 28px;
  margin-left: 0;
}
.cb li, .eb li, .ls li {
  margin-bottom: 6px;
}

/* Example box */
.eb {
  background: var(--bg2);
  border: var(--bw) solid var(--bdr);
  padding: 20px 24px;
  margin: 20px 0;
  box-shadow: var(--card-sh);
  position: relative
}

.eb::before {
  content: "";
  position: absolute;
  top: -1px;
  left: -1px;
  width: 6px;
  height: calc(100% + 2px);
  background: var(--cy)
}

.eb h4 {
  font-size: 11px;
  font-weight: 700;
  color: var(--cy);
  text-transform: uppercase;
  letter-spacing: 0.8px;
  margin-bottom: 10px
}

/* Step reveal */
.sr .st {
  padding: 10px 14px;
  font-size: 13px;
  color: var(--tx3);
  border-left: 3px solid var(--bg4);
  margin: 4px 0;
  cursor: pointer;
  font-family: var(--font);
  transition: all var(--dur-fast) var(--ease-out);
  overflow: hidden
}

.sr .st.rev {
  border-left-color: var(--ac);
  color: var(--tx);
  background: var(--bg3)
}

.sr .st:not(.rev):hover {
  background: var(--hover-bg)
}

/* Buttons */
.btn {
  padding: 12px 24px;
  font-size: 13px;
  font-weight: 600;
  letter-spacing: 0.3px;
  border: var(--bw) solid var(--bdr);
  border-radius: var(--rad);
  transition: all var(--dur) var(--ease-out)
}

.btn1 {
  background: var(--ac);
  color: var(--ac-tx);
  border-color: var(--ac);
  box-shadow: var(--shadow-sm)
}

.btn1:hover {
  box-shadow: var(--shadow);
  transform: translateY(-2px);
  filter: brightness(1.05)
}

.btn1:active {
  transform: translateY(0) scale(0.97);
  box-shadow: var(--shadow-xs);
  filter: brightness(0.95)
}

.btn2 {
  background: var(--bg3);
  color: var(--tx);
  box-shadow: var(--shadow-sm)
}

.btn2:hover {
  box-shadow: var(--shadow);
  transform: translateY(-2px)
}

.btn2:active {
  transform: translateY(0) scale(0.97);
  box-shadow: var(--shadow-xs)
}

.btns {
  padding: 8px 16px;
  font-size: 11px;
  box-shadow: var(--shadow-xs)
}

.btns:hover {
  box-shadow: var(--shadow-sm);
  transform: translateY(-1px)
}

.btns:active {
  transform: translateY(0) scale(0.97)
}

.btng {
  background: var(--gn);
  color: #0a0a0a;
  border-color: var(--gn)
}

/* Graph */
.gc {
  background: var(--bg2);
  border: var(--bw) solid var(--bdr);
  margin: 20px 0;
  box-shadow: var(--card-sh);
  overflow: hidden
}

.gc canvas {
  display: block;
  width: 100%
}

.gcc {
  display: flex;
  gap: 14px;
  padding: 12px 16px;
  align-items: center;
  background: var(--bg3);
  border-top: var(--bw) solid var(--bdr)
}

.gcc label {
  font-size: 12px;
  color: var(--tx2);
  font-family: var(--mono)
}

.gcc input[type=range] {
  width: 160px;
  accent-color: var(--ac)
}

/* ===== PRACTICE PANEL ===== */
#pp {
  position: fixed;
  right: 0;
  top: 0;
  width: 440px;
  height: 100vh;
  background: var(--bg2);
  border-left: var(--bw) solid var(--bdr);
  transform: translateX(100%);
  transition: transform var(--dur-slow) var(--ease-out);
  z-index: 100;
  display: flex;
  flex-direction: column
}

#pp.open {
  transform: none
}

.pp-h {
  padding: 20px;
  border-bottom: var(--bw) solid var(--bdr);
  display: flex;
  align-items: center;
  justify-content: space-between
}

.pp-h h3 {
  font-size: 14px;
  font-weight: 700;
  letter-spacing: 0.2px
}

#pb {
  flex: 1;
  overflow-y: auto;
  padding: 20px
}

.pp-f {
  display: flex;
  gap: 8px;
  padding: 16px;
  border-top: var(--bw) solid var(--bdr)
}

.pp-f button {
  flex: 1
}

.prp {
  font-size: 15px;
  color: var(--tx);
  margin-bottom: 16px;
  line-height: 1.8
}

.pri {
  display: flex;
  gap: 8px;
  margin-bottom: 12px
}

.pri input {
  flex: 1
}

.hb {
  background: var(--bg3);
  color: var(--tx2);
  padding: 8px 14px;
  font-size: 11px;
  font-weight: 700;
  letter-spacing: 0.2px;
  border: 2px solid var(--bdr);
  margin: 3px 3px 3px 0;
  transition: all var(--dur-fast) var(--ease-out)
}

.hb:hover {
  border-color: var(--yl);
  color: var(--yl)
}

.hc {
  background: var(--bg3);
  padding: 12px 16px;
  margin: 10px 0;
  font-size: 13px;
  color: var(--tx2);
  border-left: 4px solid var(--yl);
  line-height: 1.7
}

.fb {
  padding: 14px 18px;
  margin: 12px 0;
  font-size: 13px;
  line-height: 1.6;
  border: var(--bw) solid;
  font-weight: 500
}

.fb-ok {
  background: var(--gn-bg);
  border-color: var(--gn-bdr);
  color: var(--gn)
}

.fb-no {
  background: var(--rd-bg);
  border-color: var(--rd-bdr);
  color: var(--rd)
}

.mbar {
  display: flex;
  align-items: center;
  gap: 10px;
  margin: 14px 0
}

.mbar label {
  font-size: 10px;
  font-weight: 700;
  color: var(--tx3);
  min-width: 80px;
  letter-spacing: 0.3px;
  font-family: var(--mono)
}

.mtrk {
  flex: 1;
  height: 8px;
  background: var(--bg);
  border: 2px solid var(--bdr)
}

.mfl {
  height: 100%;
  transition: width .5s
}

.mc0 {
  background: var(--rd)
}

.mc1 {
  background: var(--or)
}

.mc2 {
  background: var(--yl)
}

.mc3 {
  background: var(--gn)
}

/* ===== MODAL ===== */
.mo {
  position: fixed;
  inset: 0;
  background: rgba(0, 0, 0, .7);
  z-index: 200;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 20px
}

[data-theme="light"] .mo {
  background: rgba(0, 0, 0, .4)
}

.md {
  background: var(--bg2);
  border: var(--bw) solid var(--bdr);
  border-radius: var(--rad-lg);
  max-width: 620px;
  width: 100%;
  max-height: 90vh;
  overflow-y: auto;
  box-shadow: var(--shadow-xl);
  animation: modalScaleIn var(--dur-slow) var(--ease-spring) both
}

.md-h {
  padding: 28px 28px 0;
  text-align: center
}

.md-h h2 {
  font-size: 28px;
  font-weight: 700;
  margin-bottom: 8px;
  letter-spacing: -1px
}

.md-h p {
  font-size: 13px;
  color: var(--tx2)
}

.md-b {
  padding: 24px 28px
}

.md-f {
  padding: 0 28px 28px;
  display: flex;
  gap: 8px;
  justify-content: center;
  flex-wrap: wrap
}

/* Quiz */
.qp {
  display: flex;
  gap: 6px;
  margin-bottom: 20px;
  justify-content: center
}

.qd {
  width: 10px;
  height: 10px;
  background: var(--bg4);
  border: 2px solid var(--bdr)
}

.qd.dn {
  background: var(--ac);
  border-color: var(--ac)
}

.qd.cu {
  background: var(--ac);
  border-color: var(--ac);
  box-shadow: 0 0 0 3px var(--bg), 0 0 0 5px var(--ac)
}

.qo {
  padding: 12px 18px;
  background: var(--bg);
  border: var(--bw) solid var(--bdr);
  font-size: 14px;
  text-align: left;
  color: var(--tx2);
  width: 100%;
  margin-bottom: 8px;
  transition: all .15s;
  box-shadow: var(--card-sh-sm)
}

.qo:hover {
  border-color: var(--ac);
  color: var(--tx);
  box-shadow: var(--shadow-sm);
  transform: translateY(-2px)
}

/* Tags */
.tag {
  display: inline-block;
  padding: 4px 10px;
  font-size: 10px;
  font-weight: 700;
  letter-spacing: 0.3px;
  margin: 2px;
  border: 2px solid var(--ac);
  color: var(--ac);
  font-family: var(--mono)
}

/* Bookmark */
.bks {
  cursor: pointer;
  font-size: 22px;
  color: var(--tx3);
  background: none;
  padding: 2px 6px;
  transition: color .2s;
  border: none
}

.bks.on {
  color: var(--yl)
}

.bks:hover {
  color: var(--yl)
}

/* Notes */
.na textarea {
  width: 100%;
  min-height: 80px;
  resize: vertical;
  font-size: 13px;
  padding: 12px
}

/* Review item */
.ri {
  display: flex;
  align-items: center;
  gap: 12px;
  padding: 14px 0;
  border-bottom: 2px solid var(--bdr)
}

.ri span {
  font-size: 12px;
  flex: 1;
  letter-spacing: 0.2px;
  font-weight: 600
}

/* Responsive */
@media(max-width:768px) {
  #sidebar {
    position: fixed;
    z-index: 50;
    height: 100vh;
    width: 85vw;
    min-width: 280px;
    max-width: 340px
  }

  /* Backdrop overlay when sidebar is open on mobile */
  #sidebar-backdrop {
    display: none;
    position: fixed;
    inset: 0;
    background: rgba(0,0,0,.5);
    z-index: 49;
    -webkit-backdrop-filter: blur(2px);
    backdrop-filter: blur(2px)
  }

  #sidebar-backdrop.active {
    display: block
  }

  #pp {
    width: 100%
  }

  #ct {
    padding: 20px 16px
  }

  #topbar {
    padding: 8px 10px;
    gap: 2px;
    overflow-x: auto;
    -webkit-overflow-scrolling: touch;
    scrollbar-width: none
  }

  #topbar::-webkit-scrollbar {
    display: none
  }

  /* 44px minimum touch targets (Apple HIG) */
  .tb-btn {
    font-size: 16px;
    padding: 8px 8px;
    min-width: 44px;
    min-height: 44px;
    display: flex;
    align-items: center;
    justify-content: center;
    flex-shrink: 0
  }

  #tbtitle {
    font-size: 13px;
    min-width: 0;
    overflow: hidden;
    text-overflow: ellipsis;
    white-space: nowrap;
    flex: 1
  }

  .theme-toggle {
    padding: 6px 10px;
    font-size: 13px;
    min-height: 44px;
    flex-shrink: 0
  }

  /* Sidebar lesson items: 44px touch targets */
  .mh {
    min-height: 44px;
    padding: 10px 12px
  }

  .li {
    min-height: 44px;
    padding: 10px 12px 10px 38px
  }

  .sb-foot button {
    min-height: 40px
  }

  /* Hero section: compact on mobile */
  .dash-hero {
    margin-bottom: 32px !important
  }

  .dash-hero h1 {
    font-size: 36px !important
  }

  .dash-hero p {
    font-size: 15px !important
  }

  /* Cards: less padding on mobile */
  .dash-card {
    padding: 20px !important
  }

  .dash-card h3 {
    font-size: 18px !important
  }

  .dash-card p {
    font-size: 13px !important
  }

  /* Practice input: ensure not hidden by keyboard */
  .pri input {
    font-size: 16px
  }

  #b-sb-close {
    display: block !important
  }
}

@media(prefers-reduced-motion:reduce) {
  * {
    transition: none !important
  }
}

/* KaTeX */
.katex {
  font-size: 1.05em !important
}

.katex-display {
  margin: 12px 0 !important;
  overflow-x: auto
}

/* Diag review cards */
.diag-card {
  border: var(--bw) solid var(--bdr);
  padding: 16px 20px;
  margin-bottom: 14px;
  background: var(--bg3);
  box-shadow: var(--card-sh-sm)
}

.diag-card.card-ok {
  border-left: 6px solid var(--gn)
}

.diag-card.card-no {
  border-left: 6px solid var(--rd)
}

.diag-step {
  font-size: 12px;
  color: var(--tx2);
  padding: 4px 0 4px 14px;
  border-left: 3px solid var(--ac);
  margin-bottom: 4px;
  line-height: 1.6
}

/* ===== MODAL ANIMATION ===== */
.mo {
  animation: modalBackdropIn var(--dur) var(--ease-out) both;
}

@keyframes modalBackdropIn {
  from { opacity: 0; }
  to { opacity: 1; }
}

@keyframes modalScaleIn {
  from { opacity: 0; transform: scale(0.92) translateY(12px); }
  to { opacity: 1; transform: scale(1) translateY(0); }
}

/* ===== SPLASH SCREEN ===== */
#splash {
  position: fixed;
  inset: 0;
  background: var(--bg);
  z-index: 9999;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  transition: opacity var(--dur-slow) var(--ease-out), visibility var(--dur-slow) var(--ease-out);
}

#splash.hidden {
  opacity: 0;
  visibility: hidden;
  pointer-events: none;
}

#splash .splash-logo {
  font-size: 72px;
  font-weight: 900;
  color: var(--ac);
  font-family: var(--mono);
  line-height: 1;
  animation: splashPulse 1.2s ease-in-out infinite;
}

#splash .splash-name {
  font-size: 24px;
  font-weight: 700;
  color: var(--tx);
  margin-top: 12px;
  letter-spacing: -0.5px;
}

#splash .splash-bar {
  width: 120px;
  height: 3px;
  background: var(--bg4);
  margin-top: 24px;
  border-radius: 3px;
  overflow: hidden;
}

#splash .splash-bar-fill {
  height: 100%;
  background: var(--ac);
  animation: splashLoad 0.8s ease-out forwards;
}

@keyframes splashPulse {
  0%, 100% { transform: scale(1); }
  50% { transform: scale(1.05); }
}

@keyframes splashLoad {
  from { width: 0; }
  to { width: 100%; }
}

/* ===== PULL-TO-REFRESH ===== */
#ptr-indicator {
  position: fixed;
  top: -50px;
  left: 50%;
  transform: translateX(-50%);
  width: 40px;
  height: 40px;
  border-radius: 50%;
  background: var(--bg2);
  border: 2px solid var(--bdr);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 18px;
  z-index: 200;
  transition: top 0.2s ease;
  box-shadow: 0 2px 8px rgba(0,0,0,.2);
}

#ptr-indicator.pulling {
  top: 12px;
}

#ptr-indicator.refreshing {
  top: 12px;
  animation: ptrSpin 0.8s linear infinite;
}

@keyframes ptrSpin {
  from { transform: translateX(-50%) rotate(0deg); }
  to { transform: translateX(-50%) rotate(360deg); }
}

/* ===== AUTH / SYNC UI ===== */
.google-signin-btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  padding: 12px 24px;
  border-radius: 6px;
  border: 1px solid var(--bdr);
  background: #fff;
  color: #3c4043;
  font-size: 14px;
  font-weight: 600;
  cursor: pointer;
  font-family: var(--sans);
  transition: all .15s;
  box-shadow: 0 1px 3px rgba(0,0,0,.08);
}

.google-signin-btn:hover {
  box-shadow: 0 2px 8px rgba(0,0,0,.15);
  border-color: #4285f4;
}

[data-theme="dark"] .google-signin-btn {
  background: #fff;
  color: #3c4043;
}

.user-avatar {
  width: 48px;
  height: 48px;
  border-radius: 50%;
  border: 2px solid var(--ac);
  object-fit: cover;
}

.sync-indicator {
  font-size: 12px;
  font-family: var(--mono);
  padding: 2px 0;
}

.sync-syncing { color: var(--yl); }
.sync-synced { color: var(--gn); }
.sync-error { color: var(--rd); }

/* ===== ENTRANCE ANIMATION ===== */
@keyframes slideUp {
  from { opacity: 0; transform: translateY(24px); }
  to { opacity: 1; transform: translateY(0); }
}

.dash-hero {
  animation: slideUp var(--dur-slow) var(--ease-out) both;
}

.dash-grid {
  animation: slideUp var(--dur-slow) var(--ease-out) 0.1s both;
}

.dash-card {
  transition: transform var(--dur) var(--ease-out), box-shadow var(--dur) var(--ease-out);
  box-shadow: var(--card-sh-sm);
}

.dash-card:hover {
  transform: translateY(-3px);
  box-shadow: var(--shadow-lg);
}

/* ===== VIEW TRANSITIONS ===== */
@keyframes viewFadeIn {
  from { opacity: 0; transform: translateY(10px); }
  to { opacity: 1; transform: translateY(0); }
}

.view-enter {
  animation: viewFadeIn var(--dur) var(--ease-out) both;
}

/* Staggered children animation */
.view-enter > *:nth-child(1) { animation-delay: 0ms; }
.view-enter > *:nth-child(2) { animation-delay: 40ms; }
.view-enter > *:nth-child(3) { animation-delay: 80ms; }
.view-enter > *:nth-child(4) { animation-delay: 120ms; }

/* ===== HINT / SOLUTION REVEAL ===== */
@keyframes revealSlide {
  from { opacity: 0; max-height: 0; transform: translateY(-8px); }
  to { opacity: 1; max-height: 500px; transform: translateY(0); }
}

.hint-reveal {
  animation: revealSlide var(--dur-slow) var(--ease-out) both;
  overflow: hidden;
}

/* ===== BADGE CELEBRATION ===== */
@keyframes celebrate {
  0% { transform: scale(0.5); opacity: 0; }
  40% { transform: scale(1.15); opacity: 1; }
  60% { transform: scale(0.95); }
  80% { transform: scale(1.05); }
  100% { transform: scale(1); opacity: 1; }
}

@keyframes confettiBurst {
  0% { opacity: 1; transform: translateY(0) scale(1); }
  100% { opacity: 0; transform: translateY(-60px) scale(0.5); }
}

.celebrate-enter {
  animation: celebrate var(--dur-slow) var(--ease-spring) both;
}

/* ===== SKELETON LOADING ===== */
.skeleton {
  background: linear-gradient(90deg, var(--bg2) 25%, var(--bg) 50%, var(--bg2) 75%);
  background-size: 200% 100%;
  animation: shimmer 1.5s infinite;
  border-radius: var(--rad);
}

@keyframes shimmer {
  0% { background-position: 200% 0; }
  100% { background-position: -200% 0; }
}

.skeleton-text {
  height: 1em;
  margin: 8px 0;
}

.skeleton-card {
  height: 120px;
  margin: 12px 0;
}

/* ===== SCREEN READER ONLY ===== */
.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0,0,0,0);
  border: 0;
}

/* ===== SAFE AREA (NOTCHED PHONES) ===== */
body {
  padding: env(safe-area-inset-top) env(safe-area-inset-right) env(safe-area-inset-bottom) env(safe-area-inset-left);
}

/* ===== ADDITIONAL BREAKPOINTS ===== */
@media(max-width: 480px) {
  #ct {
    padding: 16px 12px;
  }

  body {
    font-size: 14px;
  }

  .ls h2 {
    font-size: 22px;
  }

  .sb-head h1 {
    font-size: 14px;
  }

  #pp {
    width: 100%;
  }

  .cb, .eb {
    padding: 14px 16px;
  }

  .md {
    max-width: 100%;
  }

  .md-h {
    padding: 20px 16px 0;
  }

  .md-b {
    padding: 16px;
  }

  .md-f {
    padding: 0 16px 20px;
  }
}

@media(max-width: 360px) {
  #ct {
    padding: 12px 10px;
  }

  .ls h2 {
    font-size: 20px;
  }

  .sb-foot {
    grid-template-columns: 1fr;
  }

  .pp-f {
    flex-direction: column;
  }

  .pp-f button {
    width: 100%;
  }

  #topbar {
    flex-wrap: wrap;
    padding: 8px 12px;
    padding-left: 12px;
    gap: 6px;
  }
}

/* ===== TABLET LAYOUT (iPad portrait + landscape) ===== */
@media (min-width: 769px) and (max-width: 1199px) {
  #sidebar {
    position: relative;
    width: 280px;
    min-width: 280px;
    height: auto;
    min-height: 100vh;
    transform: none;
    z-index: 1;
    border-right: var(--bw) solid var(--bdr);
    flex-shrink: 0;
    transition: width 0.3s ease, min-width 0.3s ease;
  }
  #sidebar.collapsed {
    width: 0;
    min-width: 0;
    overflow: hidden;
    border-right: none;
  }
  #sidebar-backdrop { display: none !important; }
  #bottom-nav { display: none !important; }
  #main { padding-bottom: 0; }
  #pp {
    position: relative;
    width: 380px;
    min-width: 380px;
    height: auto;
    min-height: 100vh;
    transform: none;
    border-left: var(--bw) solid var(--bdr);
    flex-shrink: 0;
  }
  #pp:not(.open) { display: none; }
  #wrap { flex: 1; min-width: 0; }
}

@media(min-width: 1024px) {
  #ct {
    max-width: 1100px;
    padding: 40px 48px;
  }
}

@media(min-width: 1200px) {
  #ct {
    max-width: 1200px;
  }

  #sidebar {
    width: 340px;
    min-width: 340px;
  }
}

@media(min-width: 768px) {
  .dash-grid {
    grid-template-columns: repeat(2, 1fr) !important;
  }
}

@media(min-width: 1200px) {
  .dash-grid {
    grid-template-columns: repeat(2, 1fr) !important;
    gap: 32px !important;
  }
}

/* ===== TEXTBOOK LIBRARY ===== */
.tb-library {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: 24px;
  padding: 20px 0
}

.tb-book-card {
  background: var(--bg2);
  border: var(--bw) solid var(--bdr);
  border-radius: 12px;
  overflow: hidden;
  cursor: pointer;
  box-shadow: var(--card-sh);
  transition: all var(--dur) var(--ease-out)
}

.tb-book-card:hover {
  transform: translateY(-4px);
  border-color: var(--ac);
  box-shadow: var(--shadow-lg)
}

.tb-book-card:focus-visible {
  outline: 3px solid var(--ac);
  outline-offset: 2px
}

.tb-book-card-header {
  padding: 32px 24px;
  text-align: center;
  position: relative
}

.tb-book-icon {
  font-size: 56px;
  display: block;
  filter: drop-shadow(0 2px 8px rgba(0,0,0,0.3))
}

.tb-book-card-body {
  padding: 20px 24px 24px
}

.tb-book-title {
  font-size: 18px;
  font-weight: 700;
  margin-bottom: 4px;
  letter-spacing: -0.5px;
  color: var(--tx)
}

.tb-book-author {
  font-size: 12px;
  color: var(--tx3);
  margin-bottom: 10px;
  font-weight: 600;
  letter-spacing: 0.2px
}

.tb-book-desc {
  font-size: 13px;
  color: var(--tx2);
  line-height: 1.6;
  margin-bottom: 14px
}

.tb-book-meta {
  display: flex;
  gap: 12px;
  margin-bottom: 14px;
  font-size: 11px;
  color: var(--tx3);
  font-weight: 700;
  letter-spacing: 0.2px;
  font-family: var(--mono)
}

.tb-book-progress {
  display: flex;
  align-items: center;
  gap: 10px
}

.tb-book-progress .pbar {
  flex: 1;
  height: 8px
}

/* Book accent colors */
.tb-book-lang_basic_math .tb-book-title { color: #60a5fa }
.tb-book-velleman_proofs .tb-book-title { color: #4ade80 }
.tb-book-garrity_all_math .tb-book-title { color: #a78bfa }

[data-theme="light"] .tb-book-lang_basic_math .tb-book-title { color: #2563eb }
[data-theme="light"] .tb-book-velleman_proofs .tb-book-title { color: #16a34a }
[data-theme="light"] .tb-book-garrity_all_math .tb-book-title { color: #7c3aed }

/* Chapter list */
.tb-chapter-list {
  margin-top: 8px
}

.tb-chapter-item {
  margin-bottom: 4px
}

/* Section item */
.tb-section-item {
  position: relative
}

/* Breadcrumb */
.tb-breadcrumb {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 4px;
  margin-bottom: 24px;
  font-size: 12px;
  font-weight: 600;
  color: var(--tx3);
  letter-spacing: 0.2px
}

.tb-breadcrumb a {
  color: var(--ac);
  text-decoration: none;
  transition: color 0.15s
}

.tb-breadcrumb a:hover {
  color: var(--ac2);
  text-decoration: underline
}

/* Prev/Next nav */
.tb-nav-buttons {
  display: flex;
  justify-content: space-between;
  gap: 12px;
  margin-top: 32px;
  padding-top: 24px;
  border-top: 2px solid var(--bdr)
}

.tb-nav-buttons .btn {
  max-width: 45%;
  font-size: 11px;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis
}

/* Progress ring (circular) */
.tb-progress-ring {
  position: relative;
  width: 48px;
  height: 48px;
  flex-shrink: 0
}

.tb-progress-ring svg {
  transform: rotate(-90deg);
  width: 100%;
  height: 100%
}

.tb-progress-ring circle {
  fill: none;
  stroke-width: 4;
  stroke-linecap: round
}

.tb-progress-ring .ring-bg {
  stroke: var(--bg4)
}

.tb-progress-ring .ring-fg {
  transition: stroke-dashoffset 0.4s ease
}

.tb-progress-ring .ring-label {
  position: absolute;
  inset: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 10px;
  font-weight: 700;
  font-family: var(--mono);
  color: var(--tx2)
}

/* Responsive adjustments */
@media(max-width:768px) {
  .tb-library {
    grid-template-columns: 1fr
  }

  .tb-nav-buttons {
    flex-direction: column
  }

  .tb-nav-buttons .btn {
    max-width: 100%
  }

  .tb-breadcrumb {
    font-size: 10px
  }
}

@media(max-width:480px) {
  .tb-book-card-header {
    padding: 24px 16px
  }

  .tb-book-card-body {
    padding: 16px
  }

  .tb-book-icon {
    font-size: 40px
  }
}

/* ===== PRINT STYLES ===== */
@media print {
  #sidebar, #pp, .toolbar, #b-sb {
    display: none !important;
  }

  #ct {
    max-width: 100%;
    padding: 20px;
  }
}

/* Adaptive difficulty badge */
.diff-badge {
  padding: 2px 8px;
  border-radius: var(--rad);
  font-family: var(--mono);
  font-weight: 600;
  letter-spacing: 0.02em;
}
.diff-1 { background: rgba(76, 175, 80, 0.15); color: var(--gn); }
.diff-2 { background: rgba(255, 193, 7, 0.15); color: var(--yl); }
.diff-3 { background: rgba(244, 67, 54, 0.15); color: var(--rd); }

/* ── Code copy button ── */
.code-copy-btn {
  position: absolute;
  top: 6px;
  right: 6px;
  padding: 3px 10px;
  font-size: 11px;
  font-family: var(--mono);
  background: rgba(255,255,255,.08);
  color: var(--tx3);
  border: 1px solid rgba(255,255,255,.12);
  border-radius: 5px;
  cursor: pointer;
  opacity: 0;
  transition: opacity var(--dur-fast) var(--ease-out);
  z-index: 2;
}
pre:hover .code-copy-btn { opacity: 1; }
.code-copy-btn:hover { background: rgba(255,255,255,.15); color: var(--tx1); }
[data-theme="light"] .code-copy-btn {
  background: rgba(0,0,0,.06);
  color: var(--tx3);
  border-color: rgba(0,0,0,.1);
}
[data-theme="light"] pre:hover .code-copy-btn { opacity: 1; }
[data-theme="light"] .code-copy-btn:hover { background: rgba(0,0,0,.1); color: var(--tx1); }

/* ── Lab badge ── */
.lab-badge {
  display: inline-block;
  padding: 1px 7px;
  font-size: 9px;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.5px;
  border-radius: 4px;
  margin-left: 6px;
  vertical-align: middle;
}
.lab-badge.lab { background: rgba(96,165,250,.2); color: #60a5fa; }
.lab-badge.lecture { background: rgba(192,132,252,.15); color: #c084fc; }

/* ── View transitions ── */
@keyframes viewFadeIn {
  from { opacity: 0; transform: translateY(8px); }
  to { opacity: 1; transform: translateY(0); }
}
#ct > .ls, #ct > .eb, #ct > .cb, #ct > h3, #ct > .tb-breadcrumb, #ct > .tb-nav-buttons, #ct > .na {
  animation: viewFadeIn var(--dur) var(--ease-out) both;
}
#ct > *:nth-child(1) { animation-delay: 0ms; }
#ct > *:nth-child(2) { animation-delay: 40ms; }
#ct > *:nth-child(3) { animation-delay: 80ms; }
#ct > *:nth-child(4) { animation-delay: 120ms; }
#ct > *:nth-child(5) { animation-delay: 160ms; }
#ct > *:nth-child(6) { animation-delay: 200ms; }
#ct > *:nth-child(7) { animation-delay: 240ms; }
#ct > *:nth-child(8) { animation-delay: 280ms; }

/* ── Reveal animations ── */
.st {
  transition: all var(--dur) var(--ease-out);
}
.st:not(.rev) {
  cursor: pointer;
}
.st.rev {
  animation: slideReveal var(--dur) var(--ease-out) both;
}
@keyframes slideReveal {
  from { opacity: 0; transform: translateY(6px); }
  to { opacity: 1; transform: translateY(0); }
}

/* ── Modal scale-in ── */
@keyframes modalIn {
  from { opacity: 0; transform: scale(0.95) translateY(10px); }
  to { opacity: 1; transform: scale(1) translateY(0); }
}
@keyframes modalOut {
  from { opacity: 1; transform: scale(1); }
  to { opacity: 0; transform: scale(0.95); }
}
.modal-content {
  animation: modalIn var(--dur) var(--ease-spring) both;
}

/* ── Achievement celebration ── */
@keyframes celebrate {
  0% { transform: scale(0.5); opacity: 0; }
  50% { transform: scale(1.15); }
  100% { transform: scale(1); opacity: 1; }
}

/* ── XP float toast ── */
@keyframes xpFloat {
  0% { opacity: 0; transform: translateY(10px) scale(0.8); }
  60% { opacity: 1; transform: translateY(-4px) scale(1.05); }
  100% { opacity: 1; transform: translateY(0) scale(1); }
}

/* ── Loading skeleton shimmer ── */
@keyframes shimmer {
  0% { background-position: -200px 0; }
  100% { background-position: 200px 0; }
}
.skeleton {
  background: linear-gradient(90deg, var(--bg2) 25%, var(--bg3) 50%, var(--bg2) 75%);
  background-size: 400px 100%;
  animation: shimmer 1.5s infinite ease-in-out;
  border-radius: var(--rad);
}
.skeleton-line { height: 14px; margin: 8px 0; }
.skeleton-title { height: 24px; width: 60%; margin: 12px 0; }
.skeleton-block { height: 120px; margin: 12px 0; }

/* ===== GLOBAL SEARCH ===== */
.search-overlay {
  position: sticky;
  top: 0;
  z-index: 50;
  background: var(--bg);
  padding: 12px 16px;
  border-bottom: 1px solid var(--bdr);
}
.search-overlay input {
  width: 100%;
  padding: 12px 16px;
  font-size: 15px;
  border-radius: var(--rad);
  border: 2px solid var(--bdr);
  background: var(--bg2);
  color: var(--tx);
  font-family: var(--font);
}
.search-overlay input:focus {
  border-color: var(--ac);
  outline: none;
}
.search-group-title {
  font-size: 11px;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 1px;
  color: var(--tx3);
  padding: 16px 0 6px;
  border-bottom: 1px solid var(--bdr);
  margin-bottom: 8px;
}
.search-card {
  padding: 12px 0;
  border-bottom: 1px solid var(--bdr);
  cursor: pointer;
  transition: background var(--dur-fast);
}
.search-card:hover {
  background: var(--hover-bg);
}
.search-card-title {
  font-size: 14px;
  font-weight: 600;
  color: var(--tx);
  margin-bottom: 2px;
}
.search-card-source {
  font-size: 11px;
  color: var(--tx3);
  text-transform: uppercase;
  letter-spacing: 0.5px;
}
.search-card-snippet {
  font-size: 12px;
  color: var(--tx2);
  margin-top: 4px;
  line-height: 1.5;
  max-height: 40px;
  overflow: hidden;
}

/* ===== MOBILE BOTTOM NAV ===== */
#bottom-nav {
  display: none;
  position: fixed;
  bottom: 0;
  left: 0;
  right: 0;
  height: 56px;
  background: var(--bg);
  border-top: 1px solid var(--bdr);
  z-index: 1000;
  padding-bottom: env(safe-area-inset-bottom);
}
@media (max-width: 768px) {
  #bottom-nav { display: flex; justify-content: space-around; align-items: center; }
  /* Account for the nav's 56px height + the device's home-indicator safe area.
     Without this, the last ~30px of lesson content is hidden under the nav
     on notched iPhones and Android gesture-nav devices. */
  #main { padding-bottom: calc(56px + env(safe-area-inset-bottom, 0px)); }
}
#bottom-nav button {
  background: none;
  border: none;
  color: var(--tx3);
  font-size: 20px;
  padding: 8px;
  cursor: pointer;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 2px;
}
#bottom-nav button span { font-size: 9px; }
#bottom-nav button.active { color: var(--ac); }

/* ===== PRINT STYLES ===== */
/* ===== WALKTHROUGH PANEL ===== */
.wt-panel { margin-top: 12px; padding: 16px; background: var(--bg2); border-radius: var(--rad); border: 1px solid var(--bdr); }
.wt-step-text { font-size: 14px; line-height: 1.6; margin-bottom: 12px; min-height: 48px; color: var(--tx); }
.wt-controls { display: flex; gap: 8px; align-items: center; }
.wt-controls button:disabled { opacity: 0.4; cursor: not-allowed; }
.wt-step-indicator { font-size: 11px; color: var(--tx3); margin-left: auto; }

/* ===== CERTIFICATE MODAL ===== */
.cert-canvas { width: 100%; max-width: 800px; border-radius: 8px; box-shadow: var(--shadow-lg); }

@media print {
  #sidebar, #topbar, #pp, #bottom-nav, .bks, .btn, .tb-nav-buttons, .na, .sb-foot, #sidebar-backdrop { display: none !important; }
  #main { margin: 0; padding: 20px; }
  #wrap { max-width: 100%; }
  body { background: white; color: black; }
  .cb { border: 1px solid #ccc; }
  pre[class*="language-"] { background: #f5f5f5; border: 1px solid #ddd; }
}

/* ── Multiple Choice Questions ── */
.mcq-choices {
  display: flex;
  flex-direction: column;
  gap: 8px;
  margin: 12px 0;
}
.mcq-choice {
  display: flex;
  align-items: flex-start;
  gap: 12px;
  padding: 12px 16px;
  background: var(--bg2);
  border: 2px solid var(--bdr);
  border-radius: var(--rad);
  cursor: pointer;
  text-align: left;
  font-size: 14px;
  color: var(--tx);
  transition: all var(--dur-fast) var(--ease-out);
  position: relative;
}
.mcq-choice:hover:not(.mcq-answered) {
  border-color: var(--ac);
  background: rgba(200, 255, 0, .04);
  transform: translateY(-1px);
}
.mcq-letter {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 28px;
  height: 28px;
  border-radius: 50%;
  background: var(--bg3);
  color: var(--tx2);
  font-weight: 700;
  font-size: 13px;
  flex-shrink: 0;
}
.mcq-text { flex: 1; line-height: 1.5; padding-top: 2px; }
.mcq-text pre { margin: 4px 0; }

/* Answered states */
.mcq-choice.mcq-answered {
  cursor: default;
  opacity: 0.6;
  transform: none;
}
.mcq-choice.mcq-answered:hover { transform: none; border-color: var(--bdr); background: var(--bg2); }
.mcq-choice.mcq-correct {
  border-color: var(--gn) !important;
  background: rgba(76, 175, 80, .1) !important;
  opacity: 1 !important;
}
.mcq-choice.mcq-correct .mcq-letter {
  background: var(--gn);
  color: #000;
}
.mcq-choice.mcq-wrong {
  border-color: var(--rd) !important;
  background: rgba(244, 67, 54, .08) !important;
  opacity: 1 !important;
}
.mcq-choice.mcq-wrong .mcq-letter {
  background: var(--rd);
  color: #fff;
}

/* Explanation */
.mcq-explanation {
  margin: 10px 0;
  padding: 12px 16px;
  background: rgba(76, 175, 80, .06);
  border-left: 3px solid var(--gn);
  border-radius: 0 var(--rad) var(--rad) 0;
  font-size: 13px;
  color: var(--tx2);
  line-height: 1.6;
  animation: slideReveal var(--dur) var(--ease-out) both;
}

/* ===== ONBOARDING ===== */
.onboard-step { text-align: center; max-width: 500px; margin: 0 auto; padding: 40px 24px; }
.onboard-step h2 { font-size: 28px; margin-bottom: 12px; }
.onboard-step p { color: var(--tx2); font-size: 15px; line-height: 1.7; margin-bottom: 20px; }
.onboard-dots { display: flex; gap: 8px; justify-content: center; margin-bottom: 20px; }
.onboard-dot { width: 8px; height: 8px; border-radius: 50%; background: var(--bg3); }
.onboard-dot.active { background: var(--ac); }

/* ===== OFFLINE BANNER ===== */
.offline-banner {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  z-index: 9999;
  padding: 10px 20px;
  text-align: center;
  font-size: 14px;
  font-weight: 600;
  animation: offlineBannerSlide 0.3s ease;
}
.offline-banner-warn {
  background: linear-gradient(135deg, #f59e0b, #d97706);
  color: #000;
}
.offline-banner-ok {
  background: linear-gradient(135deg, #10b981, #059669);
  color: #fff;
}
@keyframes offlineBannerSlide {
  from { transform: translateY(-100%); }
  to { transform: translateY(0); }
}

/* ===== WELCOME BACK HERO ===== */
.wb-hero {
  text-align: center;
  margin-bottom: 28px;
  animation: wbFadeIn 0.6s ease;
}
.wb-greeting {
  font-size: 38px;
  font-weight: 800;
  letter-spacing: -1.5px;
  margin-bottom: 12px;
  background: linear-gradient(135deg, var(--ac), var(--cy));
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}
.wb-quote {
  font-size: 15px;
  color: var(--tx3);
  font-style: italic;
  max-width: 600px;
  margin: 0 auto;
  line-height: 1.7;
}
.wb-quote-author {
  font-style: normal;
  font-weight: 600;
  color: var(--tx2);
}

/* Motivation banner */
.wb-motivation {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 14px 20px;
  border-radius: 12px;
  margin-bottom: 20px;
  font-size: 15px;
  font-weight: 600;
  color: var(--ac);
  background: linear-gradient(135deg, rgba(0,230,118,0.06), rgba(0,188,212,0.06));
  border: 1px solid rgba(0,230,118,0.15);
  animation: wbFadeIn 0.8s ease;
}
.wb-motivation-emoji { font-size: 22px; }

/* Resume card */
.wb-resume-card {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 24px 28px;
  margin-bottom: 24px;
  border-radius: 16px;
  cursor: pointer;
  background: var(--bg2);
  border: 2px solid transparent;
  background-clip: padding-box;
  position: relative;
  box-shadow: 0 4px 24px rgba(0,0,0,0.08);
  transition: all 0.25s ease;
  animation: wbSlideUp 0.7s ease;
}
.wb-resume-card::before {
  content: '';
  position: absolute;
  inset: -2px;
  border-radius: 18px;
  background: linear-gradient(135deg, var(--ac), var(--cy), var(--ac));
  z-index: -1;
}
.wb-resume-card:hover {
  transform: translateY(-3px);
  box-shadow: 0 8px 32px rgba(0,230,118,0.15);
}
.wb-resume-left { flex: 1; min-width: 0; }
.wb-resume-module {
  font-size: 12px;
  text-transform: uppercase;
  letter-spacing: 0.5px;
  color: var(--tx3);
  margin-bottom: 6px;
  font-weight: 600;
}
.wb-resume-title {
  font-size: 22px;
  font-weight: 800;
  color: var(--tx);
  margin-bottom: 10px;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}
.wb-resume-progress-row {
  display: flex;
  align-items: center;
  gap: 10px;
}
.wb-resume-progress-track {
  flex: 1;
  height: 6px;
  background: var(--bg3);
  border-radius: 3px;
  overflow: hidden;
  max-width: 200px;
}
.wb-resume-progress-fill {
  height: 100%;
  background: linear-gradient(90deg, var(--ac), var(--cy));
  border-radius: 3px;
  transition: width 0.5s ease;
}
.wb-resume-progress-label {
  font-size: 12px;
  color: var(--tx3);
  white-space: nowrap;
}
.wb-resume-btn {
  padding: 12px 24px;
  background: linear-gradient(135deg, var(--ac), var(--cy));
  color: #000;
  font-weight: 700;
  font-size: 15px;
  border-radius: 10px;
  white-space: nowrap;
  flex-shrink: 0;
  margin-left: 20px;
  transition: transform 0.2s ease;
}
.wb-resume-card:hover .wb-resume-btn { transform: scale(1.05); }

/* Stats ribbon */
.wb-stats-ribbon {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 0;
  padding: 16px 24px;
  margin-bottom: 20px;
  border-radius: 12px;
  background: var(--bg2);
  border: 1px solid var(--bdr);
  animation: wbFadeIn 0.9s ease;
  flex-wrap: wrap;
}
.wb-stat {
  display: flex;
  align-items: center;
  gap: 6px;
  padding: 4px 16px;
}
.wb-stat-icon { font-size: 18px; }
.wb-stat-value {
  font-size: 20px;
  font-weight: 800;
  color: var(--tx);
}
.wb-stat-label {
  font-size: 11px;
  color: var(--tx3);
  text-transform: uppercase;
  letter-spacing: 0.3px;
}
.wb-stat-divider {
  width: 1px;
  height: 28px;
  background: var(--bdr);
}

/* Daily Challenge card */
.dc-card {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 20px 24px;
  margin-bottom: 20px;
  border-radius: 14px;
  background: linear-gradient(135deg, rgba(255,171,0,0.08), rgba(255,64,129,0.08));
  border: 2px solid rgba(255,171,0,0.3);
  cursor: pointer;
  transition: all 0.25s ease;
  animation: wbSlideUp 0.8s ease;
}
.dc-card:hover { transform: translateY(-2px); border-color: rgba(255,171,0,0.6); }
.dc-done { cursor: default; opacity: 0.8; }
.dc-done:hover { transform: none; }
.dc-left { flex: 1; min-width: 0; }
.dc-label {
  font-size: 12px;
  text-transform: uppercase;
  letter-spacing: 0.5px;
  color: #ffab00;
  font-weight: 700;
  margin-bottom: 4px;
}
.dc-title {
  font-size: 18px;
  font-weight: 800;
  color: var(--tx);
  text-transform: capitalize;
  margin-bottom: 4px;
}
.dc-sub {
  font-size: 12px;
  color: var(--tx3);
}
.dc-btn {
  padding: 10px 20px;
  background: linear-gradient(135deg, #ffab00, #ff6e40);
  color: #000;
  font-weight: 700;
  font-size: 14px;
  border-radius: 10px;
  white-space: nowrap;
  flex-shrink: 0;
  margin-left: 16px;
}
.dc-badge {
  font-size: 14px;
  font-weight: 700;
  color: var(--gn);
  white-space: nowrap;
  flex-shrink: 0;
  margin-left: 16px;
}

/* Confetti canvas */
.confetti-canvas {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: 99999;
  pointer-events: none;
}

/* Scratchpad tool toggle */
.hub-tool-active {
  background: var(--acc) !important;
  color: #000 !important;
  border-color: var(--acc) !important;
}

/* Math Hub */
.hub-canvas-wrap {
  border: 2px solid var(--bdr);
  border-radius: 12px;
  overflow: hidden;
  background: var(--bg2);
}
.hub-canvas-wrap canvas {
  display: block;
  width: 100%;
  height: auto;
  touch-action: none;
}
.hub-toolbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 8px;
  margin-top: 10px;
  flex-wrap: wrap;
}
.hub-toolbar-left {
  display: flex;
  align-items: center;
  gap: 6px;
}
/* Hub Solution — Textbook-quality rendering */
.hub-solution-wrap {
  animation: wbFadeIn 0.4s ease;
}
.hub-solution-header {
  font-size: 20px;
  font-weight: 800;
  margin-bottom: 20px;
  padding-bottom: 12px;
  border-bottom: 2px solid var(--bdr);
}

/* Equation card */
.hub-eq-card {
  text-align: center;
  padding: 24px;
  background: linear-gradient(135deg, rgba(200,255,0,0.04), rgba(0,188,212,0.04));
  border: 1.5px solid rgba(200,255,0,0.15);
  border-radius: 14px;
  margin-bottom: 20px;
}
.hub-eq-label {
  font-size: 10px;
  font-weight: 800;
  letter-spacing: 2px;
  text-transform: uppercase;
  color: var(--acc);
  margin-bottom: 10px;
}
.hub-eq-math {
  font-size: 22px;
  color: var(--tx);
}

/* Step cards */
.hub-step {
  display: flex;
  gap: 16px;
  padding: 18px;
  margin-bottom: 14px;
  background: var(--bg2);
  border-radius: 12px;
  border: 1px solid var(--bdr);
  transition: border-color 0.2s;
}
.hub-step:hover { border-color: var(--acc); }
.hub-step-num {
  flex-shrink: 0;
  width: 38px;
  height: 38px;
  border-radius: 50%;
  background: #c8ff00;
  color: #111;
  font-size: 16px;
  font-weight: 900;
  display: flex;
  align-items: center;
  justify-content: center;
  margin-top: 2px;
  box-shadow: 0 0 12px rgba(200,255,0,0.3);
}
.hub-step-body { flex: 1; min-width: 0; }
.hub-step-title {
  font-size: 15px;
  font-weight: 700;
  color: var(--acc);
  margin-bottom: 8px;
  text-transform: capitalize;
}
.hub-step-text {
  font-size: 14px;
  line-height: 1.8;
  color: var(--tx2);
}
.hub-step-text .katex-display,
.hub-step-text .katex {
  color: var(--tx);
  margin: 10px 0;
}

/* Final answer card */
.hub-final-card {
  text-align: center;
  padding: 28px;
  margin-top: 8px;
  margin-bottom: 8px;
  background: linear-gradient(135deg, rgba(0,230,118,0.06), rgba(0,230,118,0.02));
  border: 2px solid rgba(0,230,118,0.3);
  border-radius: 14px;
}
.hub-final-label {
  font-size: 11px;
  font-weight: 800;
  letter-spacing: 2px;
  text-transform: uppercase;
  color: var(--gn);
  margin-bottom: 12px;
}
.hub-final-math {
  font-size: 24px;
  font-weight: 600;
  color: var(--gn);
}
.hub-final-math .katex { color: var(--gn); }

/* Intro text */
.hub-intro {
  font-size: 14px;
  color: var(--tx2);
  margin-bottom: 16px;
  line-height: 1.7;
}
/* Bullet lists in steps */
.hub-step-text ul {
  margin: 10px 0;
  padding-left: 20px;
}
.hub-step-text li {
  margin-bottom: 6px;
  line-height: 1.7;
}
.hub-step-text li strong {
  display: inline;
  font-size: inherit;
  color: var(--acc);
  margin-bottom: 0;
}
.hub-final-math ul {
  list-style: none;
  padding: 0;
  text-align: center;
}
.hub-final-math li {
  margin-bottom: 8px;
}
.hub-final-math li strong {
  color: var(--gn);
}
.spinner {
  width: 28px;
  height: 28px;
  border: 3px solid var(--bdr);
  border-top-color: var(--acc);
  border-radius: 50%;
  animation: spin 0.7s linear infinite;
}
@keyframes spin { to { transform: rotate(360deg); } }

@media (max-width: 600px) {
  .dc-card { flex-direction: column; align-items: stretch; }
  .dc-btn, .dc-badge { margin-left: 0; margin-top: 10px; text-align: center; }
}

/* Animations */
@keyframes wbFadeIn {
  from { opacity: 0; }
  to { opacity: 1; }
}
@keyframes wbSlideUp {
  from { opacity: 0; transform: translateY(16px); }
  to { opacity: 1; transform: translateY(0); }
}

/* Mobile responsive */
@media (max-width: 600px) {
  .wb-greeting { font-size: 28px; }
  .wb-resume-card { flex-direction: column; align-items: stretch; padding: 20px; }
  .wb-resume-btn { margin-left: 0; margin-top: 14px; text-align: center; }
  .wb-resume-title { font-size: 18px; white-space: normal; }
  .wb-stats-ribbon { flex-wrap: wrap; gap: 4px; padding: 12px; }
  .wb-stat { padding: 4px 10px; }
  .wb-stat-divider { display: none; }
}

/* Scratchpad size picker */
.scratch-sz { cursor: pointer; opacity: 0.4; transition: opacity 0.15s, transform 0.15s; }
.scratch-sz.active { opacity: 1; transform: scale(1.2); border-color: var(--ac) !important; }
.scratch-sz:hover { opacity: 0.8; }

/* ===== TOUCH-FRIENDLY INTERACTIONS ===== */
/* Active states for tap feedback on touch devices */
.btn:active, .btns:active { transform: scale(var(--active-scale)); }
.li:active { transform: scale(var(--active-scale)); }
.mh:active { transform: scale(var(--active-scale)); }
.hb:active { transform: scale(var(--active-scale)); background: var(--bg3); }
.tb-btn:active { transform: scale(var(--active-scale)); }
.dash-card:active { transform: scale(var(--active-scale)); }
.wb-resume-card:active { transform: scale(var(--active-scale)); }
.dc-card:active { transform: scale(var(--active-scale)); }
.tb-book-card:active { transform: scale(var(--active-scale)); }
#bottom-nav button:active { transform: scale(var(--active-scale)); }
.sb-foot button:active { transform: scale(var(--active-scale)); }

/* Scope hover-only effects to pointer devices */
@media (hover: hover) {
  .btn1:hover { filter: brightness(1.1); }
  .tb-book-card:hover { transform: translateY(-4px); }
  .dash-card:hover { transform: translateY(-2px); }
  .wb-resume-card:hover { transform: translateY(-2px); }
  .dc-card:hover { transform: translateY(-2px); }
}