:root {
  --bg: #f6f8fb;
  --card: #ffffff;
  --text: #1a2332;
  --muted: #5a6a7e;
  --primary: #2563eb;
  --primary-soft: #dbeafe;
  --ok: #059669;
  --ok-bg: #ecfdf5;
  --bad: #dc2626;
  --bad-bg: #fef2f2;
  --border: #e2e8f0;
  --shadow: 0 1px 3px rgba(15, 23, 42, 0.06), 0 4px 12px rgba(15, 23, 42, 0.04);
  --radius: 12px;
  --max: 720px;
}
* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", "PingFang SC",
    "Hiragino Sans GB", "Microsoft YaHei", sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.65;
  font-size: 16px;
}
a { color: var(--primary); text-decoration: none; }
a:hover { text-decoration: underline; }
.wrap {
  max-width: var(--max);
  margin: 0 auto;
  padding: 1rem 1rem 3rem;
}
header.appbar {
  background: var(--card);
  border-bottom: 1px solid var(--border);
  position: sticky;
  top: 0;
  z-index: 20;
}
header.appbar .inner {
  max-width: var(--max);
  margin: 0 auto;
  padding: 0.75rem 1rem;
  display: flex;
  align-items: center;
  gap: 0.75rem;
  flex-wrap: wrap;
}
header.appbar .brand {
  font-weight: 700;
  color: var(--text);
  text-decoration: none;
  flex: 1;
  min-width: 0;
}
header.appbar nav { display: flex; gap: 0.5rem; flex-wrap: wrap; }
header.appbar nav a {
  padding: 0.35rem 0.7rem;
  border-radius: 8px;
  color: var(--muted);
  text-decoration: none;
  font-size: 0.9rem;
}
header.appbar nav a:hover,
header.appbar nav a.active {
  background: var(--primary-soft);
  color: var(--primary);
  text-decoration: none;
}
.hero {
  background: var(--card);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 1.75rem 1.25rem;
  margin-top: 1.25rem;
  text-align: center;
}
.hero h1 { margin: 0 0 0.4rem; font-size: 1.5rem; }
.hero p { margin: 0 0 1.25rem; color: var(--muted); }
.entries {
  display: grid;
  gap: 0.75rem;
  grid-template-columns: 1fr 1fr;
}
@media (max-width: 480px) {
  .entries { grid-template-columns: 1fr; }
}
.entry {
  display: block;
  padding: 1.1rem 1rem;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  background: #fafbfd;
  text-decoration: none;
  color: var(--text);
  transition: border-color 0.15s, background 0.15s;
}
.entry:hover {
  border-color: var(--primary);
  background: var(--primary-soft);
  text-decoration: none;
}
.entry strong { display: block; font-size: 1.1rem; margin-bottom: 0.25rem; }
.entry span { color: var(--muted); font-size: 0.9rem; }
.card {
  background: var(--card);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 1.1rem 1.15rem;
  margin-top: 1rem;
}
.filters {
  display: flex;
  flex-direction: column;
  gap: 0.65rem;
}
.filters label {
  display: flex;
  flex-direction: column;
  gap: 0.3rem;
  font-size: 0.85rem;
  color: var(--muted);
}
.filters select {
  font-size: 1rem;
  padding: 0.5rem 0.65rem;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: #fff;
  color: var(--text);
}
.row {
  display: flex;
  gap: 0.5rem;
  flex-wrap: wrap;
  align-items: center;
}
.btn {
  appearance: none;
  border: 1px solid var(--border);
  background: #fff;
  color: var(--text);
  padding: 0.55rem 0.95rem;
  border-radius: 8px;
  font-size: 0.95rem;
  cursor: pointer;
  line-height: 1.2;
}
.btn:hover { background: #f1f5f9; }
.btn:disabled { opacity: 0.45; cursor: not-allowed; }
.btn-primary {
  background: var(--primary);
  border-color: var(--primary);
  color: #fff;
}
.btn-primary:hover { background: #1d4ed8; }
.btn-ghost { background: transparent; }
.btn-ok { background: var(--ok); border-color: var(--ok); color: #fff; }
.btn-bad { background: var(--bad); border-color: var(--bad); color: #fff; }
.progress {
  font-size: 0.9rem;
  color: var(--muted);
  margin-bottom: 0.75rem;
}
.meta-tags {
  display: flex;
  flex-wrap: wrap;
  gap: 0.35rem;
  margin-bottom: 0.75rem;
}
.tag {
  font-size: 0.75rem;
  padding: 0.15rem 0.5rem;
  border-radius: 999px;
  background: #eef2ff;
  color: #3730a3;
}
.tag.source { background: #f1f5f9; color: var(--muted); }
.stem {
  font-size: 1.05rem;
  white-space: pre-wrap;
  word-break: break-word;
  margin-bottom: 1rem;
}
.options { display: flex; flex-direction: column; gap: 0.5rem; }
.opt {
  display: flex;
  gap: 0.6rem;
  align-items: flex-start;
  padding: 0.7rem 0.8rem;
  border: 1px solid var(--border);
  border-radius: 10px;
  cursor: pointer;
  background: #fff;
}
.opt:hover { border-color: #93c5fd; }
.opt.selected { border-color: var(--primary); background: var(--primary-soft); }
.opt.correct { border-color: var(--ok); background: var(--ok-bg); }
.opt.wrong { border-color: var(--bad); background: var(--bad-bg); }
.opt input { margin-top: 0.2rem; }
.opt span { white-space: pre-wrap; word-break: break-word; flex: 1; }
textarea.answer-box {
  width: 100%;
  min-height: 110px;
  padding: 0.7rem 0.8rem;
  border: 1px solid var(--border);
  border-radius: 10px;
  font: inherit;
  resize: vertical;
}
.feedback {
  margin-top: 1rem;
  padding: 0.85rem 0.95rem;
  border-radius: 10px;
  border: 1px solid var(--border);
}
.feedback.ok { background: var(--ok-bg); border-color: #a7f3d0; }
.feedback.bad { background: var(--bad-bg); border-color: #fecaca; }
.feedback h3 { margin: 0 0 0.4rem; font-size: 1rem; }
.feedback .ans, .feedback .exp {
  white-space: pre-wrap;
  word-break: break-word;
  margin: 0.35rem 0;
}
.feedback .kps { margin-top: 0.5rem; font-size: 0.9rem; color: var(--muted); }
.nav-btns {
  display: flex;
  justify-content: space-between;
  gap: 0.5rem;
  margin-top: 1rem;
  flex-wrap: wrap;
}
.stats-bar {
  font-size: 0.85rem;
  color: var(--muted);
  margin-top: 0.5rem;
}
.empty {
  text-align: center;
  color: var(--muted);
  padding: 2rem 1rem;
}
/* notes */
.notes-layout {
  display: grid;
  grid-template-columns: 200px 1fr;
  gap: 1rem;
  margin-top: 1rem;
  align-items: start;
}
@media (max-width: 700px) {
  .notes-layout { grid-template-columns: 1fr; }
}
.toc {
  background: var(--card);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 0.9rem 1rem;
  position: sticky;
  top: 60px;
  max-height: calc(100vh - 80px);
  overflow: auto;
  font-size: 0.9rem;
}
.toc h2 { margin: 0 0 0.5rem; font-size: 0.95rem; }
.toc ol { margin: 0; padding-left: 1.2rem; }
.toc a { color: var(--muted); }
.toc a:hover { color: var(--primary); }
.notes-body {
  background: var(--card);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 1.1rem 1.2rem 2rem;
}
.notes-body h1 { font-size: 1.4rem; margin-top: 0; }
.notes-body h2 {
  font-size: 1.15rem;
  margin-top: 1.6rem;
  padding-top: 0.4rem;
  border-top: 1px solid var(--border);
  scroll-margin-top: 70px;
}
.notes-body h3 { font-size: 1.05rem; }
.notes-body blockquote {
  margin: 0.8rem 0;
  padding: 0.5rem 0.85rem;
  border-left: 3px solid var(--primary);
  background: #f8fafc;
  color: var(--muted);
}
.notes-body table {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.9rem;
  margin: 0.8rem 0;
  overflow-x: auto;
  display: block;
}
.notes-body th, .notes-body td {
  border: 1px solid var(--border);
  padding: 0.4rem 0.55rem;
  text-align: left;
}
.notes-body th { background: #f1f5f9; }
.notes-body code {
  background: #f1f5f9;
  padding: 0.1rem 0.35rem;
  border-radius: 4px;
  font-size: 0.9em;
}
.notes-body hr {
  border: none;
  border-top: 1px solid var(--border);
  margin: 1.2rem 0;
}
.footer-note {
  text-align: center;
  color: var(--muted);
  font-size: 0.8rem;
  margin-top: 2rem;
}

/* Per-question free notes (collapsed by default) */
.note-panel {
  margin-top: 1rem;
  border: 1px solid var(--border);
  border-radius: 10px;
  background: #fafbfc;
  overflow: hidden;
}
.note-toggle {
  display: flex;
  align-items: center;
  justify-content: space-between;
  width: 100%;
  padding: 0.65rem 0.9rem;
  border: 0;
  background: transparent;
  cursor: pointer;
  font: inherit;
  color: var(--text);
  text-align: left;
}
.note-toggle:hover { background: #f1f5f9; }
.note-toggle .label { font-weight: 600; font-size: 0.95rem; }
.note-toggle .hint {
  font-size: 0.8rem;
  color: var(--muted);
}
.note-toggle .chevron {
  color: var(--muted);
  margin-left: 0.5rem;
  transition: transform 0.15s ease;
}
.note-panel.open .chevron { transform: rotate(90deg); }
.note-body {
  display: none;
  padding: 0 0.9rem 0.9rem;
  border-top: 1px solid var(--border);
}
.note-panel.open .note-body { display: block; }
.note-body textarea {
  width: 100%;
  min-height: 2.5em;
  margin-top: 0.75rem;
  padding: 0.65rem 0.75rem;
  border: 1px solid var(--border);
  border-radius: 8px;
  font: inherit;
  line-height: 1.55;
  resize: none;
  overflow: hidden;
  background: #fff;
  field-sizing: content; /* modern browsers */
}
.note-actions {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  margin-top: 0.6rem;
  flex-wrap: wrap;
}
.note-status {
  font-size: 0.85rem;
  color: var(--muted);
}
.note-status.saved { color: var(--ok); }
.note-dot {
  display: inline-block;
  width: 7px;
  height: 7px;
  border-radius: 50%;
  background: var(--primary);
  margin-left: 0.35rem;
  vertical-align: middle;
}