:root {
  --bg: #f4f6f9;
  --card: #fff;
  --ink: #1a2332;
  --muted: #5a6a7e;
  --line: #d8e0ea;
  --accent: #1f6feb;
  --accent-soft: #e8f0fe;
  --warn: #92400e;
  --warn-bg: #fffbeb;
  --warn-line: #fcd34d;
  --ok: #166534;
  --ok-bg: #f0fdf4;
  --ok-line: #86efac;
  --radius: 14px;
  --shadow: 0 8px 28px rgba(26, 35, 50, 0.08);
  --sans: "Noto Sans TC", "PingFang HK", "Microsoft JhengHei", system-ui, sans-serif;
}
* { box-sizing: border-box; }
body {
  margin: 0;
  font-family: var(--sans);
  color: var(--ink);
  background:
    radial-gradient(1100px 480px at 8% -8%, #dbeafe 0%, transparent 55%),
    var(--bg);
  min-height: 100vh;
  line-height: 1.55;
}
.app { max-width: 980px; margin: 0 auto; padding: 1.25rem 1rem 2.5rem; }
.topbar {
  display: flex; flex-wrap: wrap; gap: 0.75rem;
  justify-content: space-between; align-items: flex-start;
  margin-bottom: 1rem;
}
.brand h1 { margin: 0; font-size: 1.35rem; }
.brand p { margin: 0.3rem 0 0; color: var(--muted); font-size: 0.92rem; }
.lang-toggle {
  display: inline-flex; border: 1px solid var(--line); border-radius: 999px;
  overflow: hidden; background: var(--card); box-shadow: var(--shadow);
}
.lang-toggle button {
  border: 0; background: transparent; padding: 0.45rem 0.9rem;
  font: inherit; color: var(--muted); cursor: pointer;
}
.lang-toggle button.active { background: var(--accent); color: #fff; }
.panel {
  background: var(--card); border: 1px solid var(--line);
  border-radius: var(--radius); box-shadow: var(--shadow);
  padding: 1rem 1.15rem; margin-bottom: 1rem;
}
.panel h2 { margin: 0 0 0.75rem; font-size: 1.05rem; }
.intro ul { margin: 0.5rem 0 0.75rem; padding-left: 1.2rem; }
.muted { color: var(--muted); font-size: 0.88rem; }

.filter-chips {
  display: flex; flex-wrap: wrap; gap: 0.45rem;
}
.chip {
  font: inherit; border: 1px solid var(--line); background: #fff;
  border-radius: 999px; padding: 0.4rem 0.85rem; cursor: pointer;
  color: var(--muted);
}
.chip:hover { background: #f8fafc; }
.chip.active {
  background: var(--accent); border-color: var(--accent); color: #fff;
}

.topic-sections { display: flex; flex-direction: column; gap: 1.25rem; }
.cat-block h3.cat-title {
  margin: 0 0 0.65rem;
  font-size: 0.95rem;
  color: var(--muted);
  font-weight: 600;
  letter-spacing: 0.02em;
}
.dl-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
  gap: 0.85rem;
}
.dl-card {
  border: 1px solid var(--line); border-radius: 12px;
  padding: 0.85rem 0.95rem; background: #fbfdff;
  display: flex; flex-direction: column; gap: 0.35rem;
}
.dl-card h3 { margin: 0; font-size: 1rem; }
.dl-card .meta-row {
  display: flex; flex-wrap: wrap; gap: 0.4rem; align-items: center;
  margin-bottom: 0.35rem;
}
.dl-card .count { color: var(--muted); font-size: 0.85rem; }
.dl-card .paper {
  color: var(--muted); font-size: 0.78rem;
}
.badge {
  display: inline-block; font-size: 0.75rem; font-weight: 600;
  padding: 0.15rem 0.5rem; border-radius: 999px; border: 1px solid;
}
.badge.ready {
  color: var(--ok); background: var(--ok-bg); border-color: var(--ok-line);
}
.badge.preparing {
  color: var(--warn); background: var(--warn-bg); border-color: var(--warn-line);
}
.dl-card.preparing {
  opacity: 0.78;
  border-style: dashed;
  background: #f8fafc;
}
.dl-card.preparing .btns { display: none; }
.dl-card .btns { display: flex; flex-wrap: wrap; gap: 0.45rem; margin-top: 0.25rem; }
.btn {
  font: inherit; border: 1px solid var(--line); background: #fff;
  border-radius: 10px; padding: 0.4rem 0.7rem; cursor: pointer; color: var(--ink);
  text-decoration: none; display: inline-block; font-size: 0.88rem;
}
.btn:hover { background: #f8fafc; }
.btn:disabled { opacity: 0.5; cursor: not-allowed; }
.btn.primary { background: var(--accent); border-color: var(--accent); color: #fff; }
.btn.ghost { background: var(--accent-soft); border-color: #bfdbfe; color: #1e40af; }
.empty { color: var(--muted); text-align: center; padding: 1rem; }
.footer-note { text-align: center; color: var(--muted); font-size: 0.82rem; }

/* Preview view */
.dl-card.clickable .card-title {
  cursor: pointer;
  color: var(--ink);
}
.dl-card.clickable .card-title:hover {
  color: var(--accent);
  text-decoration: underline;
  text-underline-offset: 3px;
}
.preview-btn { width: 100%; text-align: center; font-weight: 600; }
.dl-row { margin-top: 0.15rem; }
.btn-sm {
  font-size: 0.78rem;
  padding: 0.3rem 0.55rem;
  border-radius: 8px;
}

.preview-view {
  padding-top: 0.85rem;
}
.preview-toolbar {
  position: sticky;
  top: 0;
  z-index: 20;
  background: rgba(255, 255, 255, 0.96);
  backdrop-filter: blur(8px);
  border-bottom: 1px solid var(--line);
  margin: -0.15rem -1.15rem 1rem;
  padding: 0.75rem 1.15rem 0.85rem;
  display: flex;
  flex-wrap: wrap;
  gap: 0.55rem 0.75rem;
  align-items: center;
}
.back-btn {
  background: var(--accent-soft);
  border-color: #bfdbfe;
  color: #1e40af;
  font-weight: 600;
}
.preview-heading {
  display: flex;
  flex-wrap: wrap;
  align-items: baseline;
  gap: 0.45rem;
  flex: 1 1 180px;
  min-width: 0;
}
.preview-heading h2 {
  margin: 0;
  font-size: 1.1rem;
}
.preview-heading .count {
  color: var(--muted);
  font-size: 0.88rem;
}
.mode-chips {
  display: flex;
  flex-wrap: wrap;
  gap: 0.35rem;
}
.mode-chip { font-size: 0.85rem; }
.preview-dl {
  display: flex;
  flex-wrap: wrap;
  gap: 0.35rem;
  margin-left: auto;
}

.preview-list {
  display: flex;
  flex-direction: column;
  gap: 0;
}
.preview-q {
  border-top: 1px solid var(--line);
  padding: 0.75rem 0;
}
.preview-q:first-child { border-top: 0; padding-top: 0.15rem; }
.preview-q h3 {
  margin: 0 0 0.25rem;
  font-size: 1rem;
  font-weight: 650;
}
.preview-stem {
  margin: 0.15rem 0 0.35rem;
  font-size: 1.05rem;
  line-height: 1.6;
}
.preview-ans,
.preview-sol {
  background: #f8fafc;
  border: 1px solid #e2e8f0;
  border-radius: 8px;
  padding: 0.65rem 0.8rem;
  margin: 0.35rem 0;
  font-size: 0.95rem;
}
.preview-sol {
  background: var(--ok-bg);
  border-color: #bbf7d0;
}

/* Offscreen host for html2pdf generation */
.pdf-export-host {
  position: fixed;
  left: -10000px;
  top: 0;
  width: 794px;
  background: #fff;
  color: #1a2332;
  font-family: "Noto Sans TC", "PingFang HK", "Microsoft JhengHei", system-ui, sans-serif;
  pointer-events: none;
  z-index: -1;
}
.pdf-export-root {
  padding: 8px 16px 24px;
  line-height: 1.55;
}
.pdf-export-root h1 {
  font-size: 1.25rem;
  margin: 0 0 0.75rem;
}
.pdf-export-root .q {
  border-top: 1px solid #d8e0ea;
  padding: 0.55rem 0;
  page-break-inside: avoid;
  break-inside: avoid;
}
.pdf-export-root .q:first-of-type { border-top: 0; }
.pdf-export-root .q h2 {
  font-size: 1rem;
  margin: 0 0 0.2rem;
  font-weight: 650;
}
.pdf-export-root .stem {
  font-size: 1.02rem;
  margin: 0;
}

@media (max-width: 640px) {
  .preview-toolbar {
    margin-left: -1rem;
    margin-right: -1rem;
    padding-left: 1rem;
    padding-right: 1rem;
  }
  .preview-dl { margin-left: 0; width: 100%; }
}