:root {
  color-scheme: light;
  --bg:           #fafafa;
  --surface:      #ffffff;
  --surface-soft: #f4f5f8;
  --text:         #1a1d23;
  --muted:        #595f6c;
  --dim:          #8a909e;
  --border:       #e2e5eb;
  --border-soft:  #ecedf1;
  --accent:       #2563eb;
  --accent-soft:  #e6effd;
  --accent-2:     #d97706;
  --accent-2-soft:#fdf3e0;
  --ok:           #16a34a;
  --shadow-sm:    0 1px 2px rgba(15, 23, 42, 0.04), 0 1px 3px rgba(15, 23, 42, 0.04);
  --shadow:       0 4px 16px rgba(15, 23, 42, 0.06);
  --shadow-lg:    0 12px 32px rgba(15, 23, 42, 0.08);
  --radius:       10px;
  --radius-lg:    14px;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }

body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  font-size: 16px;
  line-height: 1.55;
  -webkit-font-smoothing: antialiased;
}

a { color: inherit; text-decoration: none; }

/* ─── Topbar ──────────────────────────────────────────── */
.topbar {
  position: sticky;
  top: 0;
  z-index: 10;
  background: rgba(250, 250, 250, 0.85);
  backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--border);
}
.topbar-inner,
main {
  width: min(1180px, calc(100% - 36px));
  margin: 0 auto;
}
.topbar-inner {
  min-height: 56px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  font-size: 13px;
}
.brand {
  display: flex;
  align-items: baseline;
  gap: 8px;
  white-space: nowrap;
}
.brand strong {
  font-size: 15px;
  font-weight: 700;
  letter-spacing: -0.005em;
}
.brand span,
.meta {
  color: var(--muted);
}

/* ─── Hero ────────────────────────────────────────────── */
main { padding: 56px 0 96px; }

.hero {
  display: grid;
  grid-template-columns: minmax(0, 1.6fr) minmax(280px, 0.9fr);
  gap: 40px;
  align-items: end;
  margin-bottom: 56px;
}
.hero-text { min-width: 0; }
.kicker {
  margin: 0 0 16px;
  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  font-size: 12px;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--accent);
  font-weight: 600;
}
h1 {
  margin: 0;
  font-size: clamp(34px, 5.4vw, 60px);
  line-height: 1.04;
  letter-spacing: -0.018em;
  font-weight: 700;
}
.subtitle {
  max-width: 720px;
  margin: 22px 0 0;
  color: var(--muted);
  font-size: 17px;
  line-height: 1.55;
}
.hero-stats {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 10px;
}
.stat {
  padding: 14px 16px;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  background: var(--surface);
  box-shadow: var(--shadow-sm);
}
.stat .value {
  display: block;
  font-size: 26px;
  font-weight: 700;
  line-height: 1.1;
  letter-spacing: -0.01em;
}
.stat .label {
  display: block;
  margin-top: 4px;
  color: var(--muted);
  font-size: 12px;
  text-transform: uppercase;
  letter-spacing: 0.04em;
}

/* ─── Section headings ────────────────────────────────── */
.section-title {
  margin: 0 0 4px;
  font-size: 22px;
  font-weight: 700;
  letter-spacing: -0.01em;
}
.section-hint {
  margin: 0 0 18px;
  color: var(--muted);
  font-size: 14px;
}

/* ─── Quick paths ─────────────────────────────────────── */
.paths { margin-bottom: 48px; }
.path-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: 14px;
}
.path-card {
  display: grid;
  grid-template-columns: 36px minmax(0, 1fr);
  gap: 14px;
  align-items: start;
  padding: 18px 18px 20px;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  background: var(--surface);
  box-shadow: var(--shadow-sm);
  transition: border-color 0.12s ease, transform 0.12s ease, box-shadow 0.12s ease;
}
.path-card:hover {
  border-color: var(--accent);
  transform: translateY(-1px);
  box-shadow: var(--shadow);
}
.path-icon {
  width: 36px;
  height: 36px;
  display: grid;
  place-items: center;
  border-radius: 8px;
  background: var(--accent-soft);
  font-size: 18px;
}
.path-title {
  font-weight: 700;
  font-size: 15px;
  letter-spacing: -0.005em;
}
.path-meta {
  margin-top: 2px;
  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  font-size: 12px;
  color: var(--accent);
}
.path-desc {
  margin-top: 8px;
  color: var(--muted);
  font-size: 14px;
  line-height: 1.5;
}

/* ─── Search + filter chips ────────────────────────────── */
.tools {
  display: grid;
  grid-template-columns: minmax(220px, 1fr);
  gap: 12px;
  margin: 0 0 28px;
}
.search {
  width: 100%;
  min-height: 44px;
  padding: 0 14px;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  background: var(--surface);
  color: var(--text);
  font: inherit;
  outline: none;
  transition: border-color 0.12s ease, box-shadow 0.12s ease;
}
.search::placeholder { color: var(--dim); }
.search:focus {
  border-color: var(--accent);
  box-shadow: 0 0 0 3px var(--accent-soft);
}
.filter-chips {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
}
.chip {
  padding: 6px 12px;
  border: 1px solid var(--border);
  border-radius: 999px;
  background: var(--surface);
  color: var(--muted);
  font: inherit;
  font-size: 13px;
  cursor: pointer;
  transition: all 0.12s ease;
}
.chip:hover { border-color: var(--accent); color: var(--text); }
.chip[aria-pressed="true"] {
  background: var(--accent);
  border-color: var(--accent);
  color: #ffffff;
  font-weight: 600;
}

/* ─── Series grid ─────────────────────────────────────── */
.series-grid {
  display: grid;
  gap: 18px;
}
.series {
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  background: var(--surface);
  box-shadow: var(--shadow-sm);
  overflow: hidden;
}
.series-head {
  display: grid;
  grid-template-columns: minmax(0, 1fr) auto;
  gap: 24px;
  align-items: start;
  padding: 22px 24px;
}
.series-tag {
  display: inline-block;
  margin-bottom: 8px;
  padding: 2px 8px;
  border-radius: 4px;
  background: var(--accent);
  color: #ffffff;
  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  font-size: 10px;
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
}
h2 {
  margin: 0;
  font-size: 22px;
  font-weight: 700;
  line-height: 1.2;
  letter-spacing: -0.01em;
}
.series-desc {
  margin: 8px 0 0;
  color: var(--muted);
  font-size: 14.5px;
  line-height: 1.55;
  max-width: 780px;
}
.topic-row {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
  margin-top: 14px;
}
.topic {
  display: inline-block;
  padding: 3px 9px;
  border: 1px solid var(--border);
  border-radius: 4px;
  background: var(--surface-soft);
  color: var(--muted);
  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  font-size: 11.5px;
}

.series-actions {
  display: flex;
  flex-direction: column;
  align-items: flex-end;
  gap: 6px;
  flex-shrink: 0;
  min-width: 140px;
}
.series-count,
.series-time {
  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  font-size: 12px;
  color: var(--muted);
}
.series-link.primary {
  display: inline-flex;
  align-items: center;
  min-height: 32px;
  padding: 0 12px;
  margin-top: 4px;
  border-radius: 6px;
  background: var(--accent);
  color: #ffffff;
  font-size: 13px;
  font-weight: 600;
  transition: background 0.12s ease;
}
.series-link.primary:hover { background: #1d4ed8; }
.series-toggle {
  border: 1px solid var(--border);
  background: var(--surface);
  color: var(--muted);
  font: inherit;
  font-size: 12px;
  padding: 4px 10px;
  border-radius: 6px;
  cursor: pointer;
}
.series-toggle:hover { border-color: var(--accent); color: var(--text); }

/* lesson list (collapsed by default) */
.lesson-list {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 1px;
  background: var(--border-soft);
  border-top: 1px solid var(--border-soft);
}
.lesson {
  display: grid;
  grid-template-columns: 38px minmax(0, 1fr);
  gap: 12px;
  align-items: start;
  min-height: 64px;
  padding: 12px 16px;
  background: var(--surface);
}
.lesson:hover { background: var(--accent-soft); }
.num {
  width: 32px;
  height: 32px;
  display: grid;
  place-items: center;
  border-radius: 6px;
  background: var(--accent-soft);
  color: var(--accent);
  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  font-weight: 700;
  font-size: 12px;
}
.lesson-title {
  display: block;
  font-weight: 600;
  font-size: 14px;
  line-height: 1.3;
  letter-spacing: -0.005em;
}
.path {
  display: block;
  margin-top: 4px;
  color: var(--dim);
  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  font-size: 11px;
  overflow-wrap: anywhere;
}

.empty {
  display: none;
  padding: 24px;
  border: 1px dashed var(--border);
  border-radius: var(--radius);
  background: var(--surface);
  color: var(--muted);
  text-align: center;
}

.is-hidden { display: none !important; }

.footnote {
  margin-top: 56px;
  padding-top: 24px;
  border-top: 1px solid var(--border);
  text-align: center;
  color: var(--dim);
  font-size: 13px;
}

/* ─── Responsive ──────────────────────────────────────── */
@media (max-width: 900px) {
  .hero {
    grid-template-columns: 1fr;
    gap: 28px;
  }
  .series-head {
    grid-template-columns: 1fr;
  }
  .series-actions {
    flex-direction: row;
    align-items: center;
    flex-wrap: wrap;
    min-width: 0;
  }
  .lesson-list {
    grid-template-columns: 1fr;
  }
}

@media (max-width: 520px) {
  .topbar-inner,
  main {
    width: min(100% - 22px, 1180px);
  }
  .topbar-inner {
    flex-direction: column;
    align-items: flex-start;
    padding: 10px 0;
  }
  .hero-stats {
    grid-template-columns: 1fr 1fr;
  }
  .stat .value { font-size: 22px; }
  h1 { font-size: 32px; }
  .subtitle { font-size: 15px; }
  main { padding: 36px 0 72px; }
}