/* ─── Formula Reference Pages ────────────────────────────────────────────── */

/* Simple anchor-link chapter nav (formula pages use <a> not <button>) */
.chapter-nav { margin-bottom: 16px }
.chapter-nav-inner { display: flex; gap: 6px; flex-wrap: wrap }
.chapter-nav a {
  font-size: .82em;
  font-weight: 700;
  padding: 8px 16px;
  border: 2px solid var(--sand-300);
  border-radius: 10px;
  background: #fff;
  color: var(--sand-700);
  cursor: pointer;
  transition: all .2s;
  white-space: nowrap;
  text-decoration: none;
  display: inline-block;
}
.chapter-nav a:hover  { border-color: var(--sand-400); color: var(--sand-900) }
.chapter-nav a.active { background: var(--sand-900); color: #fff; border-color: var(--sand-900) }

/* Chapter sections */
.chapters { padding: 2rem 0 4rem }
.ch-section { margin-bottom: 2.5rem }
.ch-header {
  display: flex;
  align-items: center;
  gap: .75rem;
  margin-bottom: 1rem;
  padding-bottom: .6rem;
  border-bottom: 2px solid;
}
.ch-number { font-family: 'Newsreader', serif; font-size: 1.4rem; font-weight: 700; color: var(--ink-3) }
.ch-title  { font-family: 'Newsreader', serif; font-size: 1.1rem; font-weight: 600; color: var(--ink) }
.ch-marks  { font-size: .65rem; font-weight: 700; background: var(--sand-100); border: 1px solid var(--sand-200); border-radius: 999px; padding: .18rem .6rem; color: var(--ink-3); white-space: nowrap }

/* Formula card grid — cards wrap, short ones share a row */
.formula-grid {
  display: flex;
  flex-wrap: wrap;
  gap: .6rem;
}
.formula-card {
  background: #fff;
  border: 1px solid var(--sand-200);
  border-radius: 10px;
  padding: .85rem 1.1rem;
  border-left: 4px solid;
  display: flex;
  flex-wrap: wrap;
  align-items: baseline;
  gap: .25rem .75rem;
  min-width: 260px;
  flex: 1 1 auto;
}
.fc-name {
  font-size: 1rem;
  font-weight: 700;
  color: var(--ink);
  flex-shrink: 0;
}
.fc-eq {
  font-size: 1.25rem;
  color: #166534;
  background: #f0fdf4;
  border: 1px solid #86efac;
  border-radius: 5px;
  padding: .35rem .8rem;
  line-height: 1.7;
  display: inline-block;
}
.fc-note {
  font-size: .9rem;
  color: var(--ink-3);
  line-height: 1.6;
  width: 100%;
  margin-top: .25rem;
}
.fc-note strong { color: var(--ink-2) }
.unit-tag {
  display: inline-block;
  font-size: .58rem;
  font-weight: 700;
  background: var(--sand-100);
  border: 1px solid var(--sand-200);
  border-radius: 4px;
  padding: .1rem .4rem;
  color: var(--ink-3);
  font-family: monospace;
}

/* Quick-reference summary box */
.quick-ref {
  background: linear-gradient(135deg, var(--subject-bg) 0%, #fff 60%);
  border: 1.5px solid var(--subject-border);
  border-radius: 12px;
  padding: 1.25rem 1.5rem;
  margin: 1.5rem 0;
}
.quick-ref h3 {
  font-family: 'Newsreader', serif;
  font-size: .95rem;
  font-weight: 700;
  color: var(--subject-color);
  margin-bottom: .75rem;
}
.qr-grid { display: grid; grid-template-columns: repeat(auto-fit, minmax(200px, 1fr)); gap: .5rem }
.qr-item { display: flex; flex-direction: column; gap: .15rem }
.qr-label { font-size: .63rem; font-weight: 700; color: var(--ink-3); text-transform: uppercase; letter-spacing: .04em }
.qr-val {
  font-size: 1rem;
  color: var(--subject-color);
  background: var(--subject-bg);
  border: 1px solid var(--subject-border);
  border-radius: 4px;
  padding: .25rem .6rem;
  display: inline-block;
}

@media (max-width: 640px) {
  .chapter-nav {
    position: sticky;
    top: 0;
    z-index: 100;
    background: rgba(253,252,250,.95);
    backdrop-filter: blur(10px);
    margin: 0 -.75rem;
    padding: .5rem .75rem;
    border-bottom: 1px solid var(--sand-200);
  }
  .chapter-nav-inner { display: none !important; }
  .chapter-nav .mobile-select { display: block; }
  .formula-card { padding: .75rem .85rem }
  .fc-eq { font-size: .95rem }
}