:root {
  --bg: #f8fafc;
  --card: #ffffff;
  --border: rgba(15, 23, 42, 0.09);
  --text: #0f172a;
  --muted: #64748b;
  --soft: #f1f5f9;
  --accent: #7c3aed;
  --accent-soft: rgba(124, 58, 237, 0.1);
  --good: #059669;
  --good-soft: rgba(5, 150, 105, 0.1);
  --bad: #dc2626;
  --bad-soft: rgba(220, 38, 38, 0.08);
  --shadow: 0 18px 44px rgba(15, 23, 42, 0.08);
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #0b1120;
    --card: #111827;
    --border: rgba(148, 163, 184, 0.16);
    --text: #e2e8f0;
    --muted: #94a3b8;
    --soft: #1e293b;
    --accent: #a78bfa;
    --accent-soft: rgba(167, 139, 250, 0.14);
    --good: #34d399;
    --good-soft: rgba(52, 211, 153, 0.12);
    --bad: #f87171;
    --bad-soft: rgba(248, 113, 113, 0.12);
    --shadow: 0 18px 44px rgba(0, 0, 0, 0.35);
    color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --bg: #0b1120;
  --card: #111827;
  --border: rgba(148, 163, 184, 0.16);
  --text: #e2e8f0;
  --muted: #94a3b8;
  --soft: #1e293b;
  --accent: #a78bfa;
  --accent-soft: rgba(167, 139, 250, 0.14);
  --good: #34d399;
  --good-soft: rgba(52, 211, 153, 0.12);
  --bad: #f87171;
  --bad-soft: rgba(248, 113, 113, 0.12);
  --shadow: 0 18px 44px rgba(0, 0, 0, 0.35);
  color-scheme: dark;
}
* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body {
  margin: 0;
  font-family: Inter, "Noto Sans TC", ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  background: var(--bg);
  color: var(--text);
  min-height: 100vh;
  min-height: 100dvh;
  display: flex;
  flex-direction: column;
  -webkit-tap-highlight-color: transparent;
}
button { font: inherit; color: inherit; }
[hidden] { display: none !important; }

/* ---------- top bar ---------- */
.topbar {
  display: flex; align-items: center; justify-content: space-between; gap: 1rem;
  width: min(720px, 100%); margin: 0 auto;
  padding: calc(.9rem + env(safe-area-inset-top)) 1rem .6rem;
}
.brand { display: flex; align-items: center; gap: .75rem; }
.flag { display: grid; grid-template-rows: repeat(3, 1fr); width: 8px; height: 36px; border-radius: 3px; overflow: hidden; }
.flag i:nth-child(1) { background: #111; }
.flag i:nth-child(2) { background: #dd0000; }
.flag i:nth-child(3) { background: #ffce00; }
.brand h1 { margin: 0; font-size: 1.05rem; letter-spacing: -.01em; }
.brand p { margin: .1rem 0 0; font-size: .82rem; color: var(--muted); }
.icon-btn {
  display: grid; place-items: center; width: 42px; height: 42px; border-radius: 12px;
  border: 1px solid var(--border); background: var(--card); cursor: pointer;
}
.icon-btn[aria-expanded="true"] { background: var(--accent-soft); color: var(--accent); }

/* ---------- settings ---------- */
.settings {
  width: min(720px, calc(100% - 2rem)); margin: 0 auto .5rem;
  background: var(--card); border: 1px solid var(--border); border-radius: 18px;
  padding: 1rem; display: grid; gap: .85rem; box-shadow: var(--shadow);
}
.setting-row { display: grid; grid-template-columns: 4.3rem 1fr; align-items: center; gap: .6rem; }
.setting-label { font-size: .85rem; color: var(--muted); }
.seg { display: flex; flex-wrap: wrap; gap: .4rem; }
.seg button, .jump button {
  border: 1px solid var(--border); background: var(--soft); border-radius: 999px;
  padding: .42rem .72rem; font-size: .86rem; cursor: pointer;
}
.seg button.on { background: var(--accent); border-color: var(--accent); color: #fff; }
select, .jump input {
  width: 100%; border: 1px solid var(--border); background: var(--soft); color: var(--text);
  border-radius: 12px; padding: .55rem .75rem; font-size: .95rem;
}
.jump { display: flex; gap: .5rem; }
.jump input { flex: 1; min-width: 0; }
.stats-row { grid-template-columns: 1fr auto; }
.stats { font-size: .88rem; color: var(--muted); }
.stats b { color: var(--text); }
.text-btn { border: none; background: none; color: var(--bad); font-size: .85rem; cursor: pointer; padding: .3rem 0; }
.source { margin: 0; font-size: .78rem; line-height: 1.6; color: var(--muted); }
.source a { color: var(--accent); }

/* ---------- card ---------- */
.stage { flex: 1; width: min(720px, 100%); margin: 0 auto; padding: .25rem 1rem 7.5rem; }
.progress { height: 4px; border-radius: 99px; background: var(--soft); overflow: hidden; }
.progress-bar { height: 100%; width: 0; background: var(--accent); transition: width .25s ease; }
.deck-meta { display: flex; justify-content: space-between; align-items: center; margin: .55rem .15rem .6rem; font-size: .82rem; color: var(--muted); font-variant-numeric: tabular-nums; }
.badge { font-size: .76rem; padding: .18rem .55rem; border-radius: 99px; }
.badge.known { background: var(--good-soft); color: var(--good); }
.badge.learning { background: var(--bad-soft); color: var(--bad); }

.card {
  background: var(--card); border: 1px solid var(--border); border-radius: 22px;
  box-shadow: var(--shadow); padding: 1.25rem 1.15rem 1rem; cursor: pointer; outline: none;
  user-select: none; -webkit-user-select: none;
  transition: transform .18s ease, opacity .18s ease;
}
.card:focus-visible { box-shadow: 0 0 0 3px var(--accent-soft), var(--shadow); }
.card.slide-left { transform: translateX(-24px); opacity: 0; }
.card.slide-right { transform: translateX(24px); opacity: 0; }
.card-head { display: flex; justify-content: space-between; align-items: center; gap: .5rem; margin-bottom: .8rem; }
.qnum { font-size: .8rem; font-weight: 700; letter-spacing: .02em; color: var(--accent); background: var(--accent-soft); padding: .22rem .6rem; border-radius: 99px; }
.topic { font-size: .8rem; color: var(--muted); }
.question { margin: 0; font-size: 1.22rem; line-height: 1.5; font-weight: 650; }
.question-zh { margin: .55rem 0 0; font-size: 1rem; line-height: 1.65; color: var(--muted); display: none; }
.revealed .question-zh { display: block; }

.picture { margin-top: .8rem; padding: .7rem .85rem; border-radius: 14px; background: var(--soft); font-size: .88rem; line-height: 1.6; color: var(--muted); }
.picture a { color: var(--accent); }
.picture .hint { display: none; margin-top: .35rem; color: var(--text); }
.revealed .picture .hint { display: block; }

.options { list-style: none; margin: 1rem 0 0; padding: 0; display: grid; gap: .55rem; }
.option {
  display: grid; grid-template-columns: 1.9rem 1fr; gap: .6rem; align-items: start;
  padding: .75rem .8rem; border-radius: 14px; border: 1px solid var(--border); background: var(--card);
  transition: background .15s ease, border-color .15s ease;
}
.option .letter {
  display: grid; place-items: center; width: 1.9rem; height: 1.9rem; border-radius: 50%;
  background: var(--soft); font-size: .85rem; font-weight: 700;
}
.option .de { font-size: 1rem; line-height: 1.5; }
.option .zh { display: none; margin-top: .2rem; font-size: .9rem; line-height: 1.55; color: var(--muted); }
.revealed .option .zh { display: block; }
.revealed .option.correct { background: var(--good-soft); border-color: var(--good); }
.revealed .option.correct .letter { background: var(--good); color: #fff; }
.revealed .option.picked:not(.correct) { background: var(--bad-soft); border-color: var(--bad); }
.revealed .option.picked:not(.correct) .letter { background: var(--bad); color: #fff; }
.revealed .option:not(.correct):not(.picked) { opacity: .62; }

.tap-hint { margin: .95rem 0 0; text-align: center; font-size: .8rem; color: var(--muted); }
.revealed .tap-hint { display: none; }
.empty { text-align: center; color: var(--muted); padding: 3rem 1rem; line-height: 1.7; }

/* ---------- bottom actions ---------- */
.actions {
  position: fixed; left: 0; right: 0; bottom: 0;
  display: grid; grid-template-columns: auto 1fr 1fr auto; gap: .55rem;
  padding: .7rem max(1rem, calc((100% - 720px) / 2 + 1rem)) calc(.7rem + env(safe-area-inset-bottom));
  background: color-mix(in srgb, var(--bg) 88%, transparent);
  backdrop-filter: blur(12px); -webkit-backdrop-filter: blur(12px);
  border-top: 1px solid var(--border);
}
.nav-btn {
  display: grid; place-items: center; width: 50px; height: 50px; border-radius: 14px;
  border: 1px solid var(--border); background: var(--card); cursor: pointer;
}
.mark-btn { height: 50px; border-radius: 14px; font-weight: 700; font-size: .98rem; cursor: pointer; border: 1px solid transparent; }
.mark-btn.learning { background: var(--bad-soft); color: var(--bad); border-color: color-mix(in srgb, var(--bad) 30%, transparent); }
.mark-btn.known { background: var(--good); color: #fff; }
.nav-btn:disabled, .mark-btn:disabled { opacity: .4; cursor: default; }

@media (min-width: 640px) {
  .card { padding: 1.6rem 1.6rem 1.2rem; }
  .question { font-size: 1.38rem; }
}
@media (hover: hover) {
  .option:hover { border-color: var(--accent); }
  .nav-btn:hover, .icon-btn:hover { border-color: var(--accent); }
}
@media (prefers-reduced-motion: reduce) {
  .card, .progress-bar { transition: none; }
}

.toast {
  position: fixed; left: 50%; bottom: calc(5.6rem + env(safe-area-inset-bottom));
  transform: translate(-50%, 8px); opacity: 0; pointer-events: none;
  background: var(--text); color: var(--bg); padding: .55rem 1rem; border-radius: 99px;
  font-size: .88rem; transition: opacity .2s ease, transform .2s ease;
}
.toast.show { opacity: 1; transform: translate(-50%, 0); }