/* ───────────────────────── Tokens */
:root {
  color-scheme: light;
  --bg: #f6f6f3;
  --surface: #ffffff;
  --surface-2: #f0f0ec;
  --surface-3: #e7e6e1;
  --border: #e2e1dc;
  --border-strong: #cfcdc6;
  --text: #121212;
  --text-2: #4f4e4a;
  --text-3: #7a7973;
  --accent: #2a63d6;
  --accent-ink: #ffffff;
  --accent-soft: #e6eefc;
  --accent-text: #1d4fb3;
  --good: #0f8a4f;
  --good-soft: #e3f4ea;
  --bad: #c9362f;
  --bad-soft: #fbe7e5;
  --warn: #b26b00;
  --warn-soft: #fbf0dc;
  --review: #6b4fd6;
  --review-soft: #ece8fb;
  --viz: #2a78d6;
  --heat-0: #ebeae5;
  --heat-1: #c6daf5;
  --heat-2: #8db6ea;
  --heat-3: #4a8ddc;
  --heat-4: #1f5fb8;
  --shadow: 0 1px 2px rgba(20, 20, 15, 0.05), 0 4px 16px rgba(20, 20, 15, 0.05);
  --shadow-lg: 0 10px 40px rgba(20, 20, 15, 0.12);
  --radius: 14px;
  --radius-sm: 10px;
  --font: 'Inter', system-ui, -apple-system, 'Segoe UI', Roboto, sans-serif;
  --font-display: 'Space Grotesk', var(--font);
  --sidebar: 248px;
}

@media (prefers-color-scheme: dark) {
  :root:not([data-theme='light']) {
    color-scheme: dark;
    --bg: #111110;
    --surface: #1a1a19;
    --surface-2: #222220;
    --surface-3: #2c2c29;
    --border: #2e2e2b;
    --border-strong: #42423e;
    --text: #f0efec;
    --text-2: #c3c2b7;
    --text-3: #8f8e86;
    --accent: #4d86f0;
    --accent-ink: #0b0b0b;
    --accent-soft: #1d2a44;
    --accent-text: #8fb3ff;
    --good: #3cc47f;
    --good-soft: #15301f;
    --bad: #f0706a;
    --bad-soft: #3a1b19;
    --warn: #e6a23c;
    --warn-soft: #362812;
    --review: #a495f2;
    --review-soft: #272143;
    --viz: #3987e5;
    --heat-0: #262624;
    --heat-1: #1c3556;
    --heat-2: #24528a;
    --heat-3: #3277c9;
    --heat-4: #6aa6f0;
    --shadow: 0 1px 2px rgba(0, 0, 0, 0.3), 0 4px 16px rgba(0, 0, 0, 0.25);
    --shadow-lg: 0 10px 40px rgba(0, 0, 0, 0.5);
  }
}
:root[data-theme='dark'] {
  color-scheme: dark;
  --bg: #111110;
  --surface: #1a1a19;
  --surface-2: #222220;
  --surface-3: #2c2c29;
  --border: #2e2e2b;
  --border-strong: #42423e;
  --text: #f0efec;
  --text-2: #c3c2b7;
  --text-3: #8f8e86;
  --accent: #4d86f0;
  --accent-ink: #0b0b0b;
  --accent-soft: #1d2a44;
  --accent-text: #8fb3ff;
  --good: #3cc47f;
  --good-soft: #15301f;
  --bad: #f0706a;
  --bad-soft: #3a1b19;
  --warn: #e6a23c;
  --warn-soft: #362812;
  --review: #a495f2;
  --review-soft: #272143;
  --viz: #3987e5;
  --heat-0: #262624;
  --heat-1: #1c3556;
  --heat-2: #24528a;
  --heat-3: #3277c9;
  --heat-4: #6aa6f0;
  --shadow: 0 1px 2px rgba(0, 0, 0, 0.3), 0 4px 16px rgba(0, 0, 0, 0.25);
  --shadow-lg: 0 10px 40px rgba(0, 0, 0, 0.5);
}

/* ───────────────────────── Base */
*, *::before, *::after { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font: 15px/1.55 var(--font);
  -webkit-font-smoothing: antialiased;
}
a { color: var(--accent-text); text-decoration: none; }
a:hover { text-decoration: underline; }
h1, h2, h3, h4 { font-family: var(--font-display); line-height: 1.2; margin: 0; letter-spacing: -0.01em; }
h1 { font-size: 28px; font-weight: 700; }
h2 { font-size: 20px; font-weight: 650; }
h3 { font-size: 16px; font-weight: 600; }
p { margin: 0; }
button { font: inherit; color: inherit; }
:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; border-radius: 6px; }
.sr-only { position: absolute; width: 1px; height: 1px; overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; }
.muted { color: var(--text-2); }
.faint { color: var(--text-3); }
.small { font-size: 13px; }
.tiny { font-size: 12px; }
.mono { font-variant-numeric: tabular-nums; }

/* ───────────────────────── Layout */
.app { display: grid; grid-template-columns: var(--sidebar) 1fr; min-height: 100vh; }
.sidebar {
  position: sticky; top: 0; height: 100vh;
  display: flex; flex-direction: column; gap: 4px;
  padding: 20px 14px; border-right: 1px solid var(--border); background: var(--surface);
}
.brand { display: flex; align-items: center; gap: 10px; padding: 4px 10px 18px; color: var(--text); }
.brand:hover { text-decoration: none; }
.brand-mark {
  width: 34px; height: 34px; border-radius: 10px; display: grid; place-items: center;
  background: var(--accent); color: var(--accent-ink); font: 700 18px/1 var(--font-display);
}
.brand-name { font: 700 17px/1.1 var(--font-display); }
.brand-sub { font-size: 11px; color: var(--text-3); letter-spacing: 0.04em; text-transform: uppercase; }
.nav-link {
  display: flex; align-items: center; gap: 12px; padding: 9px 12px; border-radius: 10px;
  color: var(--text-2); font-weight: 500;
}
.nav-link:hover { background: var(--surface-2); color: var(--text); text-decoration: none; }
.nav-link.active { background: var(--accent-soft); color: var(--accent-text); }
.nav-link svg { width: 19px; height: 19px; flex: none; }
.nav-spacer { flex: 1; }
.nav-badge { margin-left: auto; font-size: 11px; font-weight: 600; background: var(--accent); color: var(--accent-ink); border-radius: 99px; padding: 1px 7px; }

main { min-width: 0; padding: 32px clamp(16px, 4vw, 48px) 96px; max-width: 1180px; width: 100%; }
.tabbar { display: none; }

@media (max-width: 860px) {
  .app { grid-template-columns: minmax(0, 1fr); }
  .sidebar { display: none; }
  main { padding: 20px 16px 96px; }
  .tabbar {
    display: flex; position: fixed; bottom: 0; left: 0; right: 0; z-index: 50;
    background: var(--surface); border-top: 1px solid var(--border);
    padding: 6px 4px calc(6px + env(safe-area-inset-bottom));
  }
  .tabbar a {
    flex: 1; display: flex; flex-direction: column; align-items: center; gap: 2px;
    font-size: 11px; color: var(--text-3); padding: 4px 0; border-radius: 8px;
  }
  .tabbar a:hover { text-decoration: none; }
  .tabbar a.active { color: var(--accent-text); }
  .tabbar svg { width: 22px; height: 22px; }
  h1 { font-size: 24px; }
}

/* ───────────────────────── Primitives */
.page-head { display: flex; align-items: flex-end; justify-content: space-between; gap: 16px; margin-bottom: 24px; flex-wrap: wrap; }
.page-head p { margin-top: 6px; }
.eyebrow { font-size: 12px; font-weight: 600; letter-spacing: 0.06em; text-transform: uppercase; color: var(--text-3); margin-bottom: 6px; }
.card { background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius); padding: 20px; box-shadow: var(--shadow); }
.card-flat { background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius); padding: 20px; }
.grid { display: grid; gap: 16px; }
.grid-2 { grid-template-columns: repeat(2, minmax(0, 1fr)); }
.grid-3 { grid-template-columns: repeat(3, minmax(0, 1fr)); }
.grid-4 { grid-template-columns: repeat(4, minmax(0, 1fr)); }
@media (max-width: 1000px) { .grid-4 { grid-template-columns: repeat(2, minmax(0, 1fr)); } .grid-3 { grid-template-columns: repeat(2, minmax(0, 1fr)); } }
@media (max-width: 640px) { .grid-2, .grid-3 { grid-template-columns: minmax(0, 1fr); } }
.row { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; }
.row-between { display: flex; align-items: center; justify-content: space-between; gap: 10px; }
.stack { display: flex; flex-direction: column; gap: 12px; }
.section { margin-top: 32px; }
.section-head { display: flex; align-items: baseline; justify-content: space-between; margin-bottom: 14px; gap: 12px; }

.btn {
  display: inline-flex; align-items: center; justify-content: center; gap: 8px;
  padding: 10px 16px; border-radius: 10px; border: 1px solid var(--border-strong);
  background: var(--surface); color: var(--text); font-weight: 600; font-size: 14px;
  cursor: pointer; transition: background 0.15s, border-color 0.15s, transform 0.05s; white-space: nowrap;
}
.btn:hover { background: var(--surface-2); text-decoration: none; }
.btn:active { transform: translateY(1px); }
.btn:disabled { opacity: 0.5; cursor: not-allowed; }
.btn-primary { background: var(--accent); border-color: var(--accent); color: var(--accent-ink); }
.btn-primary:hover { background: var(--accent); filter: brightness(1.08); }
.btn-ghost { border-color: transparent; background: transparent; }
.btn-ghost:hover { background: var(--surface-2); }
.btn-danger { color: var(--bad); }
.btn-sm { padding: 6px 11px; font-size: 13px; border-radius: 8px; }
.btn-lg { padding: 13px 22px; font-size: 15px; }
.btn svg { width: 17px; height: 17px; }
kbd {
  font: 600 11px/1 var(--font); padding: 3px 6px; border-radius: 5px; border: 1px solid var(--border-strong);
  border-bottom-width: 2px; color: var(--text-2); background: var(--surface-2);
}

.chip {
  display: inline-flex; align-items: center; gap: 6px; padding: 3px 10px; border-radius: 99px;
  font-size: 12px; font-weight: 600; background: var(--surface-2); color: var(--text-2); border: 1px solid var(--border);
}
.chip-accent { background: var(--accent-soft); color: var(--accent-text); border-color: transparent; }
.chip-good { background: var(--good-soft); color: var(--good); border-color: transparent; }
.chip-bad { background: var(--bad-soft); color: var(--bad); border-color: transparent; }
.chip-warn { background: var(--warn-soft); color: var(--warn); border-color: transparent; }
.diff-dots { display: inline-flex; gap: 3px; }
.diff-dots i { width: 6px; height: 6px; border-radius: 50%; background: var(--border-strong); }
.diff-dots i.on { background: var(--text-2); }

.seg { display: inline-flex; background: var(--surface-2); border: 1px solid var(--border); padding: 3px; border-radius: 11px; gap: 2px; flex-wrap: wrap; }
.seg button {
  border: 0; background: transparent; padding: 6px 12px; border-radius: 8px; font-size: 13px; font-weight: 600;
  color: var(--text-2); cursor: pointer;
}
.seg button.on { background: var(--surface); color: var(--text); box-shadow: var(--shadow); }

input[type='text'], input[type='number'], input[type='search'], select, textarea {
  font: inherit; color: var(--text); background: var(--surface); border: 1px solid var(--border-strong);
  border-radius: 10px; padding: 10px 12px; width: 100%;
}
input:focus, select:focus, textarea:focus { outline: 2px solid var(--accent); outline-offset: 0; border-color: var(--accent); }
label.field { display: flex; flex-direction: column; gap: 6px; font-size: 13px; font-weight: 600; color: var(--text-2); }

.progress { height: 6px; background: var(--surface-3); border-radius: 99px; overflow: hidden; }
.progress > span { display: block; height: 100%; background: var(--accent); border-radius: 99px; transition: width 0.4s; }

.ring { position: relative; display: inline-grid; place-items: center; }
.ring svg { transform: rotate(-90deg); }
.ring .ring-track { stroke: var(--surface-3); }
.ring .ring-val { stroke: var(--accent); transition: stroke-dashoffset 0.6s; }
.ring-label { position: absolute; text-align: center; line-height: 1.1; }
.ring-label b { display: block; font: 700 22px/1 var(--font-display); }

.empty { text-align: center; padding: 48px 20px; color: var(--text-2); }
.empty .big { font-size: 40px; margin-bottom: 10px; }

/* ───────────────────────── Dashboard */
.hero {
  display: grid; grid-template-columns: 1fr auto; gap: 24px; align-items: center;
  padding: 28px; border-radius: 18px; color: #fff;
  background: radial-gradient(120% 140% at 0% 0%, #3b6fe0 0%, #2448a8 55%, #1a2f6e 100%);
  box-shadow: var(--shadow-lg); position: relative; overflow: hidden;
}
.hero::after {
  content: '∫ Σ π θ √ ∞ Δ'; position: absolute; right: -10px; bottom: -28px; font: 700 96px/1 var(--font-display);
  color: rgba(255, 255, 255, 0.07); letter-spacing: 8px; pointer-events: none; white-space: nowrap;
}
.hero h1 { color: #fff; }
.hero p { color: rgba(255, 255, 255, 0.82); margin-top: 8px; max-width: 520px; }
.hero .btn { background: #fff; color: #1a2f6e; border-color: #fff; }
.hero .btn-outline { background: transparent; color: #fff; border-color: rgba(255, 255, 255, 0.5); }
.hero .ring .ring-track { stroke: rgba(255, 255, 255, 0.18); }
.hero .ring .ring-val { stroke: #fff; }
.hero .ring-label b { color: #fff; }
.hero .ring-label span { color: rgba(255, 255, 255, 0.8); font-size: 12px; }
@media (max-width: 640px) { .hero { grid-template-columns: minmax(0, 1fr); padding: 22px; } .hero .ring { justify-self: start; } }

.stat { display: flex; flex-direction: column; gap: 4px; }
.stat .k { font-size: 12px; font-weight: 600; color: var(--text-3); text-transform: uppercase; letter-spacing: 0.05em; }
.stat .v { font: 700 28px/1.1 var(--font-display); font-variant-numeric: tabular-nums; }
.stat .s { font-size: 13px; color: var(--text-2); }

.action-card { display: flex; gap: 14px; align-items: flex-start; color: var(--text); transition: border-color 0.15s, transform 0.15s; }
.action-card:hover { text-decoration: none; border-color: var(--accent); transform: translateY(-2px); }
.action-icon {
  width: 42px; height: 42px; border-radius: 12px; display: grid; place-items: center; flex: none;
  background: var(--accent-soft); color: var(--accent-text); font: 700 20px/1 var(--font-display);
}
.action-card h3 { margin-bottom: 3px; }

/* ───────────────────────── Syllabus */
.unit-block { margin-top: 28px; }
.unit-title { display: flex; align-items: center; gap: 10px; margin-bottom: 12px; }
.unit-icon { width: 30px; height: 30px; border-radius: 9px; display: grid; place-items: center; background: var(--surface-2); border: 1px solid var(--border); font: 700 15px/1 var(--font-display); color: var(--text-2); }
.chapter-card { display: flex; flex-direction: column; gap: 12px; color: var(--text); transition: border-color 0.15s, transform 0.15s; }
.chapter-card:hover { text-decoration: none; border-color: var(--accent); transform: translateY(-2px); }
.chapter-card .meta { display: flex; gap: 6px; flex-wrap: wrap; }

/* ───────────────────────── Chapter */
.tabs { display: flex; gap: 4px; border-bottom: 1px solid var(--border); margin-bottom: 24px; overflow-x: auto; }
.tabs a { padding: 10px 14px; color: var(--text-2); font-weight: 600; border-bottom: 2px solid transparent; margin-bottom: -1px; white-space: nowrap; }
.tabs a:hover { color: var(--text); text-decoration: none; }
.tabs a.active { color: var(--accent-text); border-bottom-color: var(--accent); }
.note-list { margin: 0; padding: 0; list-style: none; display: flex; flex-direction: column; gap: 12px; }
.note-list li { padding-left: 22px; position: relative; color: var(--text); }
.note-list li::before { content: ''; position: absolute; left: 4px; top: 10px; width: 7px; height: 7px; border-radius: 2px; background: var(--accent); transform: rotate(45deg); }
.formula { display: flex; flex-direction: column; gap: 8px; padding: 16px; border: 1px solid var(--border); border-radius: var(--radius-sm); background: var(--surface); }
.formula .name { font-size: 12px; font-weight: 600; color: var(--text-3); text-transform: uppercase; letter-spacing: 0.04em; }
.formula .tex { overflow-x: auto; overflow-y: hidden; padding: 2px 0; }
.gen-item { display: flex; align-items: center; justify-content: space-between; gap: 12px; padding: 12px 14px; border: 1px solid var(--border); border-radius: var(--radius-sm); background: var(--surface); color: var(--text); }
.gen-item:hover { border-color: var(--accent); text-decoration: none; }

/* ───────────────────────── Practice */
.session-bar { display: flex; align-items: center; justify-content: space-between; gap: 12px; margin-bottom: 18px; flex-wrap: wrap; }
.session-title { display: flex; align-items: center; gap: 10px; min-width: 0; }
.session-title h2 { white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
.q-card { max-width: 820px; }
.q-meta { display: flex; gap: 8px; align-items: center; flex-wrap: wrap; margin-bottom: 14px; }
.q-text { font-size: 17px; line-height: 1.7; overflow-x: auto; overflow-y: hidden; }
.q-text .katex-display { margin: 0.6em 0; }
.options { display: grid; gap: 10px; margin-top: 20px; }
.option {
  display: flex; align-items: center; gap: 14px; text-align: left; width: 100%;
  padding: 14px 16px; border-radius: 12px; border: 1.5px solid var(--border); background: var(--surface);
  cursor: pointer; transition: border-color 0.12s, background 0.12s; font-size: 15px; overflow-x: auto;
}
.option:hover:not(:disabled) { border-color: var(--border-strong); background: var(--surface-2); }
.option .key {
  flex: none; width: 28px; height: 28px; border-radius: 8px; display: grid; place-items: center;
  border: 1.5px solid var(--border-strong); font-weight: 700; font-size: 13px; color: var(--text-2);
}
.option.selected { border-color: var(--accent); background: var(--accent-soft); }
.option.selected .key { background: var(--accent); border-color: var(--accent); color: var(--accent-ink); }
.option.correct { border-color: var(--good); background: var(--good-soft); }
.option.correct .key { background: var(--good); border-color: var(--good); color: #fff; }
.option.wrong { border-color: var(--bad); background: var(--bad-soft); }
.option.wrong .key { background: var(--bad); border-color: var(--bad); color: #fff; }
.option:disabled { cursor: default; }
.num-input { display: flex; gap: 10px; margin-top: 20px; align-items: center; flex-wrap: wrap; }
.num-input input { max-width: 260px; font-size: 18px; font-variant-numeric: tabular-nums; }
.num-input.correct input { border-color: var(--good); background: var(--good-soft); }
.num-input.wrong input { border-color: var(--bad); background: var(--bad-soft); }
.q-actions { display: flex; gap: 10px; justify-content: space-between; align-items: center; margin-top: 22px; flex-wrap: wrap; }
.feedback { margin-top: 20px; border-radius: 12px; padding: 16px 18px; border: 1px solid var(--border); background: var(--surface-2); }
.feedback.good { background: var(--good-soft); border-color: transparent; }
.feedback.bad { background: var(--bad-soft); border-color: transparent; }
.feedback-title { display: flex; align-items: center; gap: 8px; font-weight: 700; margin-bottom: 8px; }
.feedback.good .feedback-title { color: var(--good); }
.feedback.bad .feedback-title { color: var(--bad); }
.solution { line-height: 1.75; overflow-x: auto; overflow-y: hidden; white-space: pre-line; }
.hint-box { margin-top: 16px; padding: 12px 14px; border-radius: 10px; background: var(--warn-soft); color: var(--text); border-left: 3px solid var(--warn); }
.shortcuts { margin-top: 18px; display: flex; gap: 14px; flex-wrap: wrap; color: var(--text-3); font-size: 12px; }
.icon-btn { width: 36px; height: 36px; padding: 0; border-radius: 10px; }
.icon-btn.on { color: var(--warn); }
.timer { font-variant-numeric: tabular-nums; font-weight: 600; color: var(--text-2); display: inline-flex; align-items: center; gap: 6px; }
.timer.urgent { color: var(--bad); }

/* ───────────────────────── Mock test */
.mock-layout { display: grid; grid-template-columns: minmax(0, 1fr) 280px; gap: 20px; align-items: start; }
@media (max-width: 1000px) { .mock-layout { grid-template-columns: minmax(0, 1fr); } }
.palette-panel { position: sticky; top: 20px; }
.palette { display: grid; grid-template-columns: repeat(5, 1fr); gap: 8px; margin-top: 12px; }
.pal {
  aspect-ratio: 1; border-radius: 9px; border: 1.5px solid var(--border-strong); background: var(--surface);
  font-weight: 700; font-size: 13px; cursor: pointer; display: grid; place-items: center; position: relative;
}
.pal.visited { background: var(--bad-soft); border-color: var(--bad); color: var(--bad); }
.pal.answered { background: var(--good); border-color: var(--good); color: #fff; }
.pal.marked { background: var(--review); border-color: var(--review); color: #fff; border-radius: 50%; }
.pal.answered.marked::after { content: ''; position: absolute; right: -2px; bottom: -2px; width: 10px; height: 10px; border-radius: 50%; background: var(--good); border: 2px solid var(--surface); }
.pal.current { outline: 2px solid var(--accent); outline-offset: 2px; }
.legend { display: grid; grid-template-columns: 1fr 1fr; gap: 8px; margin-top: 16px; font-size: 12px; color: var(--text-2); }
.legend span { display: flex; align-items: center; gap: 6px; }
.legend i { width: 14px; height: 14px; border-radius: 4px; border: 1.5px solid var(--border-strong); display: inline-block; }
.preset { cursor: pointer; text-align: left; border: 1.5px solid var(--border); transition: border-color 0.15s; width: 100%; }
.preset:hover { border-color: var(--border-strong); }
.preset.on { border-color: var(--accent); background: var(--accent-soft); }
.chapter-picker { display: grid; grid-template-columns: repeat(auto-fill, minmax(220px, 1fr)); gap: 8px; max-height: 280px; overflow: auto; padding: 4px; }
.check { display: flex; align-items: center; gap: 8px; font-size: 14px; padding: 6px 8px; border-radius: 8px; cursor: pointer; }
.check:hover { background: var(--surface-2); }
.check input { accent-color: var(--accent); width: 16px; height: 16px; }

.result-hero { display: grid; grid-template-columns: auto 1fr; gap: 28px; align-items: center; }
@media (max-width: 640px) { .result-hero { grid-template-columns: minmax(0, 1fr); } }
.review-item { border: 1px solid var(--border); border-radius: var(--radius-sm); background: var(--surface); }
.review-item summary { cursor: pointer; padding: 14px 16px; display: flex; align-items: center; gap: 12px; list-style: none; }
.review-item summary::-webkit-details-marker { display: none; }
.review-item .body { padding: 0 16px 16px; border-top: 1px solid var(--border); padding-top: 14px; }
.badge-dot { width: 24px; height: 24px; border-radius: 50%; display: grid; place-items: center; font-size: 12px; font-weight: 800; flex: none; }
.badge-dot.good { background: var(--good-soft); color: var(--good); }
.badge-dot.bad { background: var(--bad-soft); color: var(--bad); }
.badge-dot.skip { background: var(--surface-2); color: var(--text-3); }
.review-q { flex: 1; min-width: 0; overflow: hidden; white-space: nowrap; text-overflow: ellipsis; color: var(--text); }

/* ───────────────────────── Flashcards */
.flash-wrap { perspective: 1400px; max-width: 640px; margin: 0 auto; }
.flash {
  position: relative; height: 300px; transform-style: preserve-3d; transition: transform 0.5s cubic-bezier(0.2, 0.7, 0.2, 1);
  cursor: pointer;
}
.flash.flipped { transform: rotateY(180deg); }
.flash-face {
  position: absolute; inset: 0; backface-visibility: hidden; -webkit-backface-visibility: hidden;
  display: flex; flex-direction: column; align-items: center; justify-content: center; gap: 14px; text-align: center;
  padding: 28px; border-radius: 18px; background: var(--surface); border: 1px solid var(--border); box-shadow: var(--shadow-lg);
}
.flash-back { transform: rotateY(180deg); }
.flash-face .front-text { font: 600 22px/1.35 var(--font-display); }
.flash-face .back-tex { font-size: clamp(12px, 3.5vw, 22px); width: 100%; min-width: 0; }
@media (max-width: 640px) { .flash-face { padding: 20px 14px; } }
.flash-face .back-text { font-size: 17px; line-height: 1.6; max-width: 100%; overflow-wrap: anywhere; }
.flash-face .back-tex .katex-display { overflow-x: auto; overflow-y: hidden; padding: 4px 0; margin: 0; }
.formula .tex .katex-display, .q-text .katex-display, .solution .katex-display { overflow-x: auto; overflow-y: hidden; }
.grade-row { display: grid; grid-template-columns: repeat(4, 1fr); gap: 10px; max-width: 640px; margin: 20px auto 0; }
.grade-row .btn { flex-direction: column; gap: 2px; padding: 10px 6px; }
.grade-row .btn small { font-weight: 500; color: var(--text-3); font-size: 11px; }
.grade-again { color: var(--bad); }
.grade-easy { color: var(--good); }

/* ───────────────────────── Analytics */
.bars { display: flex; flex-direction: column; gap: 10px; }
.bar-row { display: grid; grid-template-columns: minmax(120px, 220px) 1fr 64px; gap: 12px; align-items: center; font-size: 14px; }
.bar-row .label { white-space: nowrap; overflow: hidden; text-overflow: ellipsis; color: var(--text); }
.bar-row .val { text-align: right; font-variant-numeric: tabular-nums; color: var(--text-2); font-size: 13px; }
.bar-track { height: 12px; background: var(--surface-2); border-radius: 4px; overflow: hidden; }
.bar-fill { height: 100%; background: var(--viz); border-radius: 0 4px 4px 0; }
.bar-row:hover .bar-fill { filter: brightness(1.12); }
@media (max-width: 640px) { .bar-row { grid-template-columns: 110px 1fr 48px; gap: 8px; } }
.heatmap { display: grid; grid-auto-flow: column; grid-template-rows: repeat(7, 12px); gap: 3px; overflow-x: auto; padding-bottom: 4px; }
.heatmap i { width: 12px; height: 12px; border-radius: 3px; background: var(--heat-0); }
.heatmap i[data-l='1'] { background: var(--heat-1); }
.heatmap i[data-l='2'] { background: var(--heat-2); }
.heatmap i[data-l='3'] { background: var(--heat-3); }
.heatmap i[data-l='4'] { background: var(--heat-4); }
.heat-legend { display: flex; align-items: center; gap: 4px; font-size: 12px; color: var(--text-3); margin-top: 10px; }
.heat-legend i { width: 12px; height: 12px; border-radius: 3px; display: inline-block; }
.chart-svg { width: 100%; height: auto; display: block; overflow: visible; }
.chart-svg .grid-line { stroke: var(--border); stroke-width: 1; }
.chart-svg .axis-text { fill: var(--text-3); font-size: 11px; font-family: var(--font); }
.chart-svg .line { stroke: var(--viz); stroke-width: 2; fill: none; stroke-linejoin: round; stroke-linecap: round; }
.chart-svg .area { fill: var(--viz); opacity: 0.1; }
.chart-svg .dot { fill: var(--viz); stroke: var(--surface); stroke-width: 2; }
.chart-wrap { position: relative; }
.tooltip {
  position: absolute; pointer-events: none; background: var(--text); color: var(--bg); padding: 6px 10px; border-radius: 8px;
  font-size: 12px; white-space: nowrap; transform: translate(-50%, calc(-100% - 10px)); opacity: 0; transition: opacity 0.12s; z-index: 5;
}
.tooltip.show { opacity: 1; }
table.data { width: 100%; border-collapse: collapse; font-size: 14px; }
table.data th, table.data td { padding: 9px 10px; border-bottom: 1px solid var(--border); text-align: left; }
table.data th { font-size: 12px; color: var(--text-3); font-weight: 600; text-transform: uppercase; letter-spacing: 0.04em; }
table.data td.n, table.data th.n { text-align: right; font-variant-numeric: tabular-nums; }
.table-scroll { overflow-x: auto; }

/* ───────────────────────── Toast & modal */
.toasts { position: fixed; bottom: 24px; right: 24px; display: flex; flex-direction: column; gap: 8px; z-index: 100; }
@media (max-width: 860px) { .toasts { bottom: 84px; right: 16px; left: 16px; } }
.toast {
  background: var(--text); color: var(--bg); padding: 11px 16px; border-radius: 10px; font-size: 14px; font-weight: 500;
  box-shadow: var(--shadow-lg); animation: toast-in 0.25s ease-out;
}
@keyframes toast-in { from { opacity: 0; transform: translateY(8px); } }
.confetti { position: fixed; inset: 0; pointer-events: none; z-index: 99; overflow: hidden; }
.confetti i { position: absolute; top: -12px; width: 8px; height: 12px; border-radius: 2px; animation: fall 1.6s ease-in forwards; }
@keyframes fall { to { transform: translateY(105vh) rotate(720deg); opacity: 0.7; } }

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { transition: none !important; animation: none !important; }
}

/* ───────────────────────── Print: formula sheets */
@media print {
  .sidebar, .tabbar, .tabs, .no-print, .toasts { display: none !important; }
  .app { display: block; }
  main { padding: 0; max-width: none; }
  body { background: #fff; color: #000; }
  .card, .formula { box-shadow: none; break-inside: avoid; }
}