/* ============================================================================
   BlinkNBuild Hardware Academy - 3-Column Layout & Ad Sanctuary
   ----------------------------------------------------------------------------
   Grid:  [ 280px sidebar ] [ 860px reading column ] [ 300px ad rail ]

   AD SANCTUARY RULES ENFORCED HERE:
     1. The centre reading column NEVER contains a sticky/floating ad.
     2. Desktop display ads live only in .ad-rail (right, outside reading flow).
     3. The rail is display:none below 1280px - no mobile right rail, ever.
     4. Every ad slot reserves its height up front so there is ZERO layout
        shift (CLS) when AdSense fills or fails to fill.
   ========================================================================== */

/* ============================ Top navbar ================================ */

.academy-nav {
  position: sticky;
  top: 0;
  z-index: 100;
  height: var(--navbar-h);
  background: rgba(10, 14, 26, 0.88);
  backdrop-filter: blur(14px);
  -webkit-backdrop-filter: blur(14px);
  border-bottom: 1px solid var(--border-subtle);
}

.academy-nav-inner {
  max-width: 1680px;
  height: 100%;
  margin: 0 auto;
  padding: 0 var(--sp-5);
  display: flex;
  align-items: center;
  gap: var(--sp-5);
}

.academy-brand {
  display: flex;
  align-items: center;
  gap: var(--sp-2);
  min-width: 0;
  flex: 0 1 auto;
}

.academy-brand img { flex-shrink: 0; }

.academy-brand img { width: 32px; height: 32px; }

.academy-brand-text {
  font-family: var(--font-heading);
  font-size: 15px;
  font-weight: 700;
  color: var(--text-primary);
  letter-spacing: -0.02em;
}

.academy-brand-text span { color: var(--cyan-400); }

.academy-brand-sub {
  font-family: var(--font-code);
  font-size: 9.5px;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--text-faint);
  display: block;
  margin-top: -2px;
}

.academy-nav-links {
  display: flex;
  align-items: center;
  gap: var(--sp-5);
  margin: 0 0 0 var(--sp-4);
  list-style: none;
}

.academy-nav-links a {
  font-family: var(--font-subheading);
  font-size: 14px;
  font-weight: 500;
  color: var(--text-tertiary);
}

.academy-nav-links a:hover,
.academy-nav-links a.is-active { color: var(--cyan-300); }

.academy-nav-right {
  margin-left: auto;
  display: flex;
  align-items: center;
  gap: var(--sp-3);
  min-width: 0;
  flex: 0 1 auto;
}

/* Search box */
.toc-search-wrap {
  position: relative;
  min-width: 0;
  flex: 0 1 auto;
}

.toc-search {
  width: 210px;
  max-width: 100%;
  background: var(--bg-card);
  border: 1px solid var(--border-soft);
  border-radius: var(--r-full);
  color: var(--text-primary);
  font-family: var(--font-body);
  font-size: 13.5px;
  padding: 7px 14px 7px 34px;
  transition: all var(--dur-base) var(--ease-out);
}

.toc-search::placeholder { color: var(--text-faint); }
.toc-search:focus {
  outline: none;
  border-color: var(--border-cyan);
  box-shadow: 0 0 0 3px rgba(0, 212, 245, 0.1);
}

.toc-search-icon {
  position: absolute;
  left: 13px;
  top: 50%;
  transform: translateY(-50%);
  font-size: 12px;
  color: var(--text-faint);
  pointer-events: none;
}

/* Mobile sidebar toggle */
.sidebar-toggle {
  display: none;
  background: var(--bg-card);
  border: 1px solid var(--border-soft);
  border-radius: var(--r-md);
  color: var(--text-secondary);
  font-size: 15px;
  width: 38px;
  height: 38px;
  cursor: pointer;
  align-items: center;
  justify-content: center;
}

/* ============================ Shell grid ================================ */

.academy-shell {
  position: relative;   /* deliberately no z-index: see the drawer note below */
  max-width: 1680px;
  margin: 0 auto;
  padding: 0 var(--sp-5);
  display: grid;
  grid-template-columns: var(--sidebar-w) minmax(0, 1fr) var(--ad-rail-w);
  gap: var(--sp-8);
  align-items: start;
}

/* --------------------------- Left sidebar ------------------------------- */

.academy-sidebar {
  position: sticky;
  top: var(--navbar-h);
  height: calc(100vh - var(--navbar-h));
  overflow-y: auto;
  overscroll-behavior: contain;
  padding: var(--sp-6) var(--sp-2) var(--sp-8) 0;
  scrollbar-width: thin;
  scrollbar-color: var(--neutral-400) transparent;
}

.academy-sidebar::-webkit-scrollbar { width: 6px; }
.academy-sidebar::-webkit-scrollbar-thumb {
  background: var(--neutral-400);
  border-radius: var(--r-full);
}
.academy-sidebar::-webkit-scrollbar-track { background: transparent; }

.toc-heading {
  font-family: var(--font-code);
  font-size: 10.5px;
  font-weight: 600;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--text-faint);
  padding: 0 var(--sp-3);
  margin-bottom: var(--sp-3);
}

/* Progress bar */
.progress-wrap {
  margin: 0 var(--sp-3) var(--sp-5);
  padding: var(--sp-3) var(--sp-4);
  background: var(--bg-surface);
  border: 1px solid var(--border-subtle);
  border-radius: var(--r-md);
}

.progress-label {
  display: flex;
  justify-content: space-between;
  align-items: center;
  font-family: var(--font-code);
  font-size: 10.5px;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  color: var(--text-faint);
  margin-bottom: var(--sp-2);
}

.progress-label b { color: var(--cyan-300); font-weight: 600; }

.progress-track {
  height: 5px;
  background: var(--neutral-300);
  border-radius: var(--r-full);
  overflow: hidden;
}

.progress-fill {
  height: 100%;
  width: 0;
  background: linear-gradient(90deg, var(--cyan-500), var(--cyan-300));
  border-radius: var(--r-full);
  transition: width var(--dur-slow) var(--ease-out);
}

/* Volume groups */
.toc-vol { margin-bottom: var(--sp-1); }

.toc-vol > summary {
  cursor: pointer;
  list-style: none;
  display: flex;
  align-items: center;
  gap: var(--sp-2);
  padding: 8px var(--sp-3);
  border-radius: var(--r-sm);
  font-family: var(--font-subheading);
  font-size: 13.5px;
  font-weight: 600;
  color: var(--text-secondary);
  transition: all var(--dur-fast) var(--ease-out);
}

.toc-vol > summary::-webkit-details-marker { display: none; }
.toc-vol > summary:hover { background: var(--bg-surface); color: var(--cyan-200); }

.toc-vol > summary::before {
  content: "\f105"; /* chevron-right */
  font-family: "Font Awesome 6 Free";
  font-weight: 900;
  font-size: 11px;
  width: 10px;
  flex-shrink: 0;
  color: var(--text-faint);
  transition: transform var(--dur-base) var(--ease-out);
}

.toc-vol[open] > summary::before { transform: rotate(90deg); }
.toc-vol[open] > summary { color: var(--cyan-300); }

.toc-vol-num {
  font-family: var(--font-code);
  font-size: 10px;
  color: var(--text-faint);
  flex-shrink: 0;
}

.toc-sublist {
  list-style: none;
  margin: 2px 0 var(--sp-2) 0;
  padding-left: var(--sp-6);
  border-left: 1px solid var(--border-subtle);
  margin-left: var(--sp-4);
}

.toc-sublist li { margin: 0; }

.toc-sublist a {
  display: block;
  padding: 6px var(--sp-3);
  border-radius: var(--r-sm);
  font-size: 13px;
  line-height: 1.45;
  color: var(--text-tertiary);
  border-left: 2px solid transparent;
  margin-left: -1px;
}

.toc-sublist a:hover { color: var(--cyan-200); background: var(--bg-surface); }

.toc-sublist a.is-current {
  color: var(--cyan-300);
  background: rgba(0, 212, 245, 0.07);
  border-left-color: var(--cyan-400);
  font-weight: 600;
}

/* Locked / not-yet-published entries */
.toc-sublist a.is-locked { color: var(--text-faint); pointer-events: none; opacity: 0.55; }
.toc-sublist a.is-locked::after {
  content: "\f0c1"; /* link */
  font-family: "Font Awesome 6 Free";
  font-weight: 900;
  font-size: 9px;
  margin-left: 6px;
  opacity: 0.5;
}

/* No-results state for search filter */
.toc-empty {
  display: none;
  padding: var(--sp-4) var(--sp-3);
  font-size: 13px;
  color: var(--text-faint);
  font-style: italic;
}

.academy-sidebar.is-filtered-empty .toc-empty { display: block; }

/* --------------------------- Centre column ------------------------------ */

.academy-main {
  min-width: 0;
  max-width: var(--content-max-w);
  padding: var(--sp-8) 0 var(--sp-12);
}

.breadcrumb {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: var(--sp-2);
  list-style: none;
  margin: 0 0 var(--sp-5);
  font-family: var(--font-code);
  font-size: 11.5px;
  color: var(--text-faint);
}

.breadcrumb li { margin: 0; display: flex; align-items: center; gap: var(--sp-2); }
.breadcrumb li + li::before { content: "/"; color: var(--neutral-400); }
.breadcrumb a { color: var(--text-tertiary); }
.breadcrumb a:hover { color: var(--cyan-300); }
.breadcrumb [aria-current="page"] { color: var(--cyan-400); }

.lesson-header {
  padding-bottom: var(--sp-6);
  margin-bottom: var(--sp-6);
  border-bottom: 1px solid var(--border-subtle);
}

.lesson-meta {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: var(--sp-3);
  margin-bottom: var(--sp-4);
}

.lesson-meta-item {
  font-family: var(--font-code);
  font-size: 11.5px;
  color: var(--text-faint);
  display: inline-flex;
  align-items: center;
  gap: 6px;
}

.lesson-lede {
  font-family: var(--font-subheading);
  font-size: 17.5px;
  line-height: 1.7;
  color: var(--text-secondary);
  margin-bottom: 0;
}

/* Prev / next pager */
.lesson-pager {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--sp-4);
  margin-top: var(--sp-10);
  padding-top: var(--sp-6);
  border-top: 1px solid var(--border-subtle);
}

.pager-link {
  display: block;
  padding: var(--sp-4) var(--sp-5);
  background: var(--bg-surface);
  border: 1px solid var(--border-subtle);
  border-radius: var(--r-md);
  transition: all var(--dur-base) var(--ease-out);
}

.pager-link:hover { border-color: var(--border-cyan); background: var(--bg-card); }
.pager-link.is-next { text-align: right; }
.pager-link.is-disabled { opacity: 0.35; pointer-events: none; }

.pager-dir {
  display: block;
  font-family: var(--font-code);
  font-size: 10.5px;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--text-faint);
  margin-bottom: 4px;
}

.pager-title {
  font-family: var(--font-subheading);
  font-size: 14.5px;
  font-weight: 600;
  color: var(--text-primary);
}

/* ============================ Google Auto Ads =========================== */
/* Google Auto Ads automatically discovers optimal placements across the page.
   These rules ensure ad units render cleanly without layout breaks or empty boxes. */

.google-auto-placed {
  margin: var(--sp-6) auto;
  max-width: 100%;
  clear: both;
}

ins.adsbygoogle[data-ad-status="unfilled"] {
  display: none !important;
  height: 0 !important;
  margin: 0 !important;
}

/* Sponsor card (direct B2B sponsorship, not AdSense) */
.sponsor-card {
  padding: var(--sp-4);
  background: var(--bg-card);
  border: 1px solid var(--border-cyan);
  border-radius: var(--r-md);
  font-size: 13px;
}

.sponsor-card .sponsor-tag {
  font-family: var(--font-code);
  font-size: 9px;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--text-faint);
  display: block;
  margin-bottom: var(--sp-2);
}

/* ============================ Footer ==================================== */

.academy-footer {
  position: relative;
  margin-top: var(--sp-12);
  padding: var(--sp-8) var(--sp-5);
  border-top: 1px solid var(--border-subtle);
  background: var(--bg-surface);
}

.academy-footer-inner {
  max-width: 1680px;
  margin: 0 auto;
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: space-between;
  gap: var(--sp-4);
  font-size: 13px;
  color: var(--text-tertiary);
}

.academy-footer-links {
  display: flex;
  flex-wrap: wrap;
  gap: var(--sp-5);
  list-style: none;
  margin: 0;
}

.academy-footer-links a { color: var(--text-tertiary); }
.academy-footer-links a:hover { color: var(--cyan-300); }

.academy-footer-disclaimer {
  max-width: 1680px;
  margin: var(--sp-4) auto 0;
  padding-top: var(--sp-3);
  border-top: 1px solid var(--border-subtle);
  font-size: 11.5px;
  line-height: 1.6;
  color: var(--text-faint);
}

/* ============================ Responsive ================================ */

@media (max-width: 1279px) {
  .ad-rail { display: none; }
}

/* --- Below 1024px: sidebar becomes an off-canvas drawer ------------------ */
@media (max-width: 1023px) {
  .academy-shell {
    grid-template-columns: minmax(0, 1fr);
    padding: 0 var(--sp-4);
  }

  .sidebar-toggle { display: inline-flex; }
  .toc-search { width: 150px; }
  .academy-nav-links { display: none; }

  /* The drawer and its scrim live in the ROOT stacking context. Never add a
     z-index to .academy-shell - doing so traps this element below the scrim. */
  .academy-sidebar {
    position: fixed;
    top: var(--navbar-h);
    left: 0;
    z-index: 90;
    width: min(320px, 86vw);
    height: calc(100vh - var(--navbar-h));
    background: var(--bg-space);
    border-right: 1px solid var(--border-soft);
    padding: var(--sp-5) var(--sp-3) var(--sp-8);
    transform: translateX(-102%);
    transition: transform var(--dur-slow) var(--ease-out);
    box-shadow: 12px 0 40px rgba(0, 0, 0, 0.5);
  }

  .academy-sidebar.is-open { transform: translateX(0); }

  .sidebar-scrim {
    position: fixed;
    inset: var(--navbar-h) 0 0 0;
    z-index: 89;
    background: rgba(8, 13, 26, 0.7);
    backdrop-filter: blur(2px);
    opacity: 0;
    visibility: hidden;
    transition: opacity var(--dur-base) var(--ease-out), visibility var(--dur-base);
  }

  .sidebar-scrim.is-open { opacity: 1; visibility: visible; }

  .academy-main { max-width: none; padding-top: var(--sp-6); }

  /* Mobile ad policy: ONE banner between major sections + one before footer */
  
}

@media (max-width: 640px) {
  body { font-size: 15.5px; }

  .academy-shell { padding: 0 var(--sp-4); }
  .academy-brand-sub { display: none; }
  .toc-search { width: 120px; }

  figure { padding: var(--sp-3); border-radius: var(--r-md); }
  .callout { grid-template-columns: 20px 1fr; padding: var(--sp-3) var(--sp-4); font-size: 14.5px; }
  .formula { padding: var(--sp-4) var(--sp-3); font-size: 13.5px; }

  .lesson-pager { grid-template-columns: 1fr; }
  .lesson-pager .is-next { text-align: left; }
}

/* ============================ Print ===================================== */

@media print {
  .academy-nav, .academy-sidebar, .ad-rail, .ad-slot,
  .sidebar-scrim, .lesson-pager, .copy-btn { display: none !important; }

  .academy-shell { display: block; padding: 0; }
  body { background: #fff; color: #000; }
  body::before { display: none; }
}

/* ============================================================================
   Hub / catalog variant - no left sidebar, ad rail retained
   ========================================================================== */

.academy-shell--hub {
  grid-template-columns: 1fr;
  max-width: 1280px;
}

.academy-shell--hub .academy-main { max-width: none; }

@media (max-width: 1279px) {
  .academy-shell--hub { grid-template-columns: minmax(0, 1fr); }
}

/* --- Hub hero ----------------------------------------------------------- */

.hub-hero {
  padding: var(--sp-10) 0 var(--sp-8);
  border-bottom: 1px solid var(--border-subtle);
  margin-bottom: var(--sp-9);
}

.hub-eyebrow {
  display: inline-flex;
  align-items: center;
  gap: var(--sp-2);
  font-family: var(--font-code);
  font-size: 11px;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--cyan-400);
  margin-bottom: var(--sp-4);
}

.hub-hero h1 { margin-bottom: var(--sp-4); }

.hub-hero p {
  font-family: var(--font-subheading);
  font-size: 17.5px;
  line-height: 1.7;
  max-width: 720px;
}

.hub-stats {
  display: flex;
  flex-wrap: wrap;
  gap: var(--sp-8);
  margin-top: var(--sp-7);
}

.hub-stat-num {
  font-family: var(--font-heading);
  font-size: 26px;
  font-weight: 900;
  color: var(--cyan-300);
  line-height: 1.1;
}

.hub-stat-label {
  font-family: var(--font-code);
  font-size: 10.5px;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--text-faint);
  margin-top: 4px;
}

/* --- Course cards -------------------------------------------------------- */

.course-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(320px, 1fr));
  gap: var(--sp-5);
  margin: var(--sp-6) 0 var(--sp-10);
}

.course-card {
  display: flex;
  flex-direction: column;
  padding: var(--sp-6);
  background: var(--bg-surface);
  border: 1px solid var(--border-subtle);
  border-radius: var(--r-lg);
  transition: all var(--dur-base) var(--ease-out);
}

a.course-card:hover {
  border-color: var(--border-cyan);
  background: var(--bg-card);
  transform: translateY(-3px);
  box-shadow: 0 14px 38px rgba(0, 0, 0, 0.35);
}

.course-card.is-soon { opacity: 0.72; }

.course-card-top {
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: var(--sp-3);
  margin-bottom: var(--sp-4);
}

.course-icon {
  width: 44px;
  height: 44px;
  display: flex;
  align-items: center;
  justify-content: center;
  border-radius: var(--r-md);
  background: rgba(0, 212, 245, 0.1);
  border: 1px solid var(--border-cyan);
  color: var(--cyan-300);
  font-size: 19px;
  flex-shrink: 0;
}

.course-card h3 {
  font-family: var(--font-heading);
  font-size: 17px;
  color: var(--text-primary);
  margin: 0 0 var(--sp-2);
  line-height: 1.35;
}

.course-card p {
  font-size: 14.5px;
  color: var(--text-tertiary);
  margin-bottom: var(--sp-5);
  flex-grow: 1;
}

.course-card-meta {
  display: flex;
  flex-wrap: wrap;
  gap: var(--sp-2) var(--sp-4);
  padding-top: var(--sp-4);
  border-top: 1px solid var(--border-subtle);
  font-family: var(--font-code);
  font-size: 11.5px;
  color: var(--text-faint);
}

.course-card-meta span {
  display: inline-flex;
  align-items: center;
  gap: 6px;
}

/* --- Volume list on the course overview page ---------------------------- */

.vol-list { list-style: none; margin: var(--sp-6) 0; padding: 0; }

.vol-item {
  display: flex;
  align-items: flex-start;
  gap: var(--sp-4);
  padding: var(--sp-4) var(--sp-5);
  margin-bottom: var(--sp-3);
  background: var(--bg-surface);
  border: 1px solid var(--border-subtle);
  border-radius: var(--r-md);
  transition: all var(--dur-base) var(--ease-out);
}

a.vol-item:hover {
  border-color: var(--border-cyan);
  background: var(--bg-card);
  transform: translateX(4px);
}

.vol-item.is-soon { opacity: 0.6; }

.vol-num {
  font-family: var(--font-heading);
  font-size: 13px;
  font-weight: 900;
  color: var(--cyan-500);
  padding-top: 2px;
  flex-shrink: 0;
  width: 30px;
}

.vol-body { min-width: 0; flex-grow: 1; }

.vol-title {
  font-family: var(--font-subheading);
  font-size: 15.5px;
  font-weight: 600;
  color: var(--text-primary);
  display: block;
  margin-bottom: 3px;
}

.vol-desc { font-size: 13.5px; color: var(--text-tertiary); line-height: 1.6; }

.vol-tag { flex-shrink: 0; align-self: center; }

/* --- Mark-complete control ---------------------------------------------- */

.mark-complete {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  font-family: var(--font-subheading);
  font-size: 13.5px;
  font-weight: 600;
  padding: 9px 16px;
  background: var(--bg-card);
  border: 1px solid var(--border-soft);
  border-radius: var(--r-md);
  color: var(--text-tertiary);
  cursor: pointer;
  transition: all var(--dur-base) var(--ease-out);
}

.mark-complete:hover { border-color: var(--border-cyan); color: var(--cyan-300); }

.mark-complete.is-complete {
  border-color: rgba(0, 230, 118, 0.4);
  color: var(--emerald);
  background: rgba(0, 230, 118, 0.08);
}

/* Completed volume marker in the sidebar */
.toc-vol.is-done > summary { color: var(--emerald); }

.toc-vol.is-done > summary .toc-vol-num::after {
  content: "00c"; /* check */
  font-family: "Font Awesome 6 Free";
  font-weight: 900;
  font-size: 9px;
  margin-left: 5px;
  color: var(--emerald);
}

/* ============================================================================
   Rapid-revision components (Volume 10)
   ========================================================================== */

/* --- Formula reference cards -------------------------------------------- */

.formula-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(310px, 1fr));
  gap: var(--sp-4);
  margin: var(--sp-6) 0;
}

.formula-card {
  display: flex;
  flex-direction: column;
  padding: var(--sp-4) var(--sp-5);
  background: var(--bg-surface);
  border: 1px solid var(--border-subtle);
  border-left: 3px solid var(--cyan-400);
  border-radius: var(--r-md);
}

.formula-card .fc-title {
  font-family: var(--font-subheading);
  font-size: 13px;
  font-weight: 700;
  color: var(--text-primary);
  margin-bottom: var(--sp-3);
}

.formula-card .fc-eq {
  font-family: var(--font-code);
  font-size: 13px;
  line-height: 1.85;
  color: var(--cyan-200);
  overflow-x: auto;
  padding-bottom: 2px;
  flex-grow: 1;
}

.formula-card .fc-eq em {
  color: var(--text-faint);
  font-style: normal;
  font-size: 11.5px;
}

.formula-card .fc-ref {
  margin-top: var(--sp-3);
  padding-top: var(--sp-2);
  border-top: 1px solid var(--border-subtle);
  font-family: var(--font-code);
  font-size: 10.5px;
  color: var(--text-faint);
}

.formula-card .fc-ref a { color: var(--text-tertiary); }
.formula-card .fc-ref a:hover { color: var(--cyan-300); }

.formula-card.is-gold    { border-left-color: var(--gold-400); }
.formula-card.is-gold .fc-eq { color: var(--gold-200); }
.formula-card.is-emerald { border-left-color: var(--emerald); }
.formula-card.is-purple  { border-left-color: var(--violet-300); }
.formula-card.is-purple .fc-eq { color: var(--violet-200); }

/* --- Flashcards ---------------------------------------------------------- */

.deck-controls {
  display: flex;
  flex-wrap: wrap;
  gap: var(--sp-3);
  align-items: center;
  margin: var(--sp-6) 0 var(--sp-4);
}

.deck-btn {
  display: inline-flex;
  align-items: center;
  gap: 7px;
  font-family: var(--font-subheading);
  font-size: 13px;
  font-weight: 600;
  padding: 8px 15px;
  background: var(--bg-card);
  border: 1px solid var(--border-soft);
  border-radius: var(--r-md);
  color: var(--text-tertiary);
  cursor: pointer;
  transition: all var(--dur-base) var(--ease-out);
}

.deck-btn:hover { border-color: var(--border-cyan); color: var(--cyan-300); }

.deck-count {
  font-family: var(--font-code);
  font-size: 11.5px;
  color: var(--text-faint);
  margin-left: auto;
}

.flashcard-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(320px, 1fr));
  gap: var(--sp-3);
  margin: var(--sp-4) 0 var(--sp-8);
}

.flashcard {
  background: var(--bg-surface);
  border: 1px solid var(--border-subtle);
  border-radius: var(--r-md);
  overflow: hidden;
  transition: border-color var(--dur-base) var(--ease-out);
}

.flashcard[open] { border-color: var(--border-cyan); }

.flashcard > summary {
  cursor: pointer;
  list-style: none;
  display: flex;
  align-items: flex-start;
  gap: var(--sp-3);
  padding: var(--sp-3) var(--sp-4);
  font-family: var(--font-body);
  font-size: 14px;
  line-height: 1.5;
  color: var(--text-secondary);
}

.flashcard > summary::-webkit-details-marker { display: none; }
.flashcard > summary:hover { background: var(--bg-card); color: var(--text-primary); }

.flashcard .fc-num {
  flex-shrink: 0;
  font-family: var(--font-code);
  font-size: 10px;
  font-weight: 600;
  color: var(--cyan-500);
  background: rgba(0, 212, 245, 0.08);
  border: 1px solid var(--border-cyan);
  border-radius: var(--r-sm);
  padding: 2px 6px;
  margin-top: 1px;
}

.flashcard[open] > summary { border-bottom: 1px solid var(--border-subtle); }

.flashcard-answer {
  padding: var(--sp-3) var(--sp-4) var(--sp-4) calc(var(--sp-4) + 34px);
  font-size: 13.5px;
  line-height: 1.65;
  color: var(--cyan-200);
}

.flashcard-answer code {
  font-size: 0.88em;
}

/* --- Drill numbering ----------------------------------------------------- */

.drill-num {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 22px;
  height: 22px;
  flex-shrink: 0;
  border-radius: var(--r-full);
  background: rgba(255, 82, 82, 0.14);
  border: 1px solid rgba(255, 82, 82, 0.32);
  color: var(--crimson);
  font-family: var(--font-code);
  font-size: 10.5px;
  font-weight: 600;
}

@media (max-width: 640px) {
  .formula-grid,
  .flashcard-grid { grid-template-columns: 1fr; }
  .deck-count { margin-left: 0; width: 100%; }
}