/*
 * Practice — Filterable problem index + per-problem reveal-solution UI.
 * File: assets/css/practice.css
 *
 * Designed to live alongside about.css. Reuses the same accent palette so
 * the practice section feels native to the rest of the site.
 *
 * All classes are prefixed `pr-` to avoid collisions with theme styles.
 * Works in both light and dark mode via Chirpy CSS vars + local fallbacks.
 */

/* ════════════════════════════════════════════════
   0. PALETTE — reused from about.css so the design language is shared.
════════════════════════════════════════════════ */
:root {
  --pr-c-blue:   #3b82f6;
  --pr-c-violet: #8b5cf6;
  --pr-c-green:  #10b981;
  --pr-c-amber:  #f59e0b;
  --pr-c-red:    #ef4444;
  --pr-c-teal:   #14b8a6;
  --pr-c-pink:   #ec4899;
  --pr-c-orange: #f97316;

  --pr-easy-fg:   #047857;
  --pr-easy-bg:   rgba(16,185,129,.12);
  --pr-medium-fg: #b45309;
  --pr-medium-bg: rgba(245,158,11,.12);
  --pr-hard-fg:   #b91c1c;
  --pr-hard-bg:   rgba(239,68,68,.12);

  --pr-card-bg:    var(--card-bg, #ffffff);
  --pr-card-shadow: 0 1px 2px rgba(0,0,0,.04), 0 1px 0 rgba(0,0,0,.02);
  --pr-card-shadow-hover: 0 8px 24px rgba(0,0,0,.08), 0 2px 0 rgba(0,0,0,.02);
  --pr-border: var(--main-border-color, rgba(0,0,0,.08));
  --pr-text-muted: var(--text-muted-color, #6b7280);

  --pr-chip-bg:        rgba(99,102,241,.07);
  --pr-chip-border:    rgba(99,102,241,.18);
  --pr-chip-active-bg: linear-gradient(135deg, #3b82f6 0%, #8b5cf6 100%);
  --pr-chip-active-fg: #ffffff;
}

@media (prefers-color-scheme: dark) {
  :root:not([data-mode="light"]) {
    --pr-easy-fg:   #34d399;
    --pr-easy-bg:   rgba(52,211,153,.16);
    --pr-medium-fg: #fbbf24;
    --pr-medium-bg: rgba(251,191,36,.16);
    --pr-hard-fg:   #f87171;
    --pr-hard-bg:   rgba(248,113,113,.16);

    --pr-card-bg: #1f2630;
    --pr-card-shadow: 0 1px 0 rgba(255,255,255,.02), 0 1px 2px rgba(0,0,0,.4);
    --pr-card-shadow-hover: 0 8px 24px rgba(0,0,0,.5), 0 2px 0 rgba(255,255,255,.02);
    --pr-border: rgba(255,255,255,.08);
    --pr-text-muted: #9ca3af;

    --pr-chip-bg:        rgba(139,92,246,.10);
    --pr-chip-border:    rgba(139,92,246,.28);
  }
}
[data-mode="dark"] {
  --pr-easy-fg:   #34d399;
  --pr-easy-bg:   rgba(52,211,153,.16);
  --pr-medium-fg: #fbbf24;
  --pr-medium-bg: rgba(251,191,36,.16);
  --pr-hard-fg:   #f87171;
  --pr-hard-bg:   rgba(248,113,113,.16);

  --pr-card-bg: #1f2630;
  --pr-card-shadow: 0 1px 0 rgba(255,255,255,.02), 0 1px 2px rgba(0,0,0,.4);
  --pr-card-shadow-hover: 0 8px 24px rgba(0,0,0,.5), 0 2px 0 rgba(255,255,255,.02);
  --pr-border: rgba(255,255,255,.08);
  --pr-text-muted: #9ca3af;

  --pr-chip-bg:        rgba(139,92,246,.10);
  --pr-chip-border:    rgba(139,92,246,.28);
}

/* ════════════════════════════════════════════════
   1. HERO
════════════════════════════════════════════════ */
.pr-hero {
  position: relative;
  margin: 0 0 2rem;
  padding: 2.5rem 2rem 2.25rem;
  border-radius: 18px;
  background:
    radial-gradient(900px 320px at 0% 0%, rgba(59,130,246,.10), transparent 60%),
    radial-gradient(700px 240px at 100% 100%, rgba(139,92,246,.10), transparent 60%),
    var(--pr-card-bg);
  border: 1px solid var(--pr-border);
  overflow: hidden;
}
.pr-hero-inner { width: 100%; }

.pr-eyebrow {
  display: inline-block;
  font-size: .75rem;
  font-weight: 600;
  letter-spacing: .12em;
  text-transform: uppercase;
  color: var(--pr-c-blue);
  background: rgba(59,130,246,.10);
  padding: .3rem .65rem;
  border-radius: 999px;
  margin-bottom: 1rem;
}
.pr-title {
  font-size: clamp(1.85rem, 1.4rem + 1.6vw, 2.6rem);
  font-weight: 800;
  line-height: 1.15;
  margin: 0 0 .65rem;
  letter-spacing: -.02em;
  background: linear-gradient(135deg, #3b82f6 0%, #8b5cf6 50%, #ec4899 100%);
  -webkit-background-clip: text;
  background-clip: text;
  -webkit-text-fill-color: transparent;
}
.pr-subtitle {
  font-size: 1.05rem;
  line-height: 1.6;
  color: var(--pr-text-muted);
  margin: 0 0 1.5rem;
  width: 100%;
}

.pr-stats {
  display: flex;
  gap: .9rem;
  flex-wrap: wrap;
  margin-top: 1.25rem;
}
.pr-stat {
  display: flex;
  flex-direction: column;
  align-items: flex-start;
  padding: .75rem 1.1rem;
  background: rgba(0,0,0,.025);
  border: 1px solid var(--pr-border);
  border-radius: 12px;
  min-width: 90px;
}
[data-mode="dark"] .pr-stat { background: rgba(255,255,255,.025); }
@media (prefers-color-scheme: dark) {
  :root:not([data-mode="light"]) .pr-stat { background: rgba(255,255,255,.025); }
}
.pr-stat strong {
  font-size: 1.5rem;
  font-weight: 800;
  line-height: 1;
  color: var(--heading-color, inherit);
}
.pr-stat span {
  font-size: .72rem;
  text-transform: uppercase;
  letter-spacing: .08em;
  color: var(--pr-text-muted);
  margin-top: .3rem;
}
.pr-stat-link { text-decoration: none; transition: transform .15s ease; }
.pr-stat-link:hover { transform: translateY(-2px); }
.pr-stat-link strong { color: var(--pr-c-blue); }

/* ════════════════════════════════════════════════
   2. TOOLBAR (search + chip filters)
════════════════════════════════════════════════ */
.pr-toolbar {
  display: flex;
  flex-direction: column;
  gap: .6rem;
  padding: .75rem .9rem;
  background: var(--pr-card-bg);
  border: 1px solid var(--pr-border);
  border-radius: 12px;
  margin-bottom: 1rem;
  position: sticky;
  top: .75rem;
  z-index: 5;
  backdrop-filter: blur(8px);
}

.pr-toolbar-row {
  display: flex;
  align-items: center;
  gap: .6rem;
  flex-wrap: wrap;
}

.pr-search {
  position: relative;
  display: flex;
  align-items: center;
  flex: 1 1 220px;
  min-width: 200px;
}
.pr-search svg {
  position: absolute;
  left: .85rem;
  width: 18px;
  height: 18px;
  color: var(--pr-text-muted);
  pointer-events: none;
}
.pr-search input {
  width: 100%;
  padding: .55rem .85rem .55rem 2.3rem;
  border: 1px solid var(--pr-border);
  border-radius: 8px;
  background: transparent;
  color: inherit;
  font-size: .9rem;
  font-family: inherit;
  transition: border-color .15s ease, box-shadow .15s ease;
}
.pr-search input:focus {
  outline: none;
  border-color: var(--pr-c-blue);
  box-shadow: 0 0 0 3px rgba(59,130,246,.18);
}
.pr-clear-btn {
  position: absolute;
  right: .55rem;
  width: 26px;
  height: 26px;
  border-radius: 50%;
  border: 0;
  background: rgba(0,0,0,.05);
  color: var(--pr-text-muted);
  font-size: 1.2rem;
  line-height: 1;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 0;
}
[data-mode="dark"] .pr-clear-btn { background: rgba(255,255,255,.05); }
.pr-clear-btn:hover { background: rgba(59,130,246,.15); color: var(--pr-c-blue); }

.pr-chip-group {
  display: flex;
  flex-wrap: wrap;
  gap: .35rem;
  align-items: center;
}
.pr-chip-difficulty { flex-shrink: 0; }
.pr-chip-categories {
  width: 100%;
  border-top: 1px solid var(--pr-border);
  padding-top: .55rem;
}

.pr-chip {
  display: inline-flex;
  align-items: center;
  gap: .35rem;
  padding: .3rem .7rem;
  font-size: .8rem;
  font-weight: 500;
  background: var(--pr-chip-bg);
  border: 1px solid var(--pr-chip-border);
  border-radius: 999px;
  color: inherit;
  cursor: pointer;
  transition: background .15s ease, border-color .15s ease, color .15s ease, transform .15s ease;
  user-select: none;
  font-family: inherit;
  line-height: 1.2;
}
.pr-chip:hover {
  border-color: var(--pr-c-blue);
  transform: translateY(-1px);
}
.pr-chip.is-active {
  background: var(--pr-chip-active-bg);
  color: var(--pr-chip-active-fg);
  border-color: transparent;
  box-shadow: 0 2px 8px rgba(59,130,246,.3);
}

.pr-chip-count {
  font-size: .68rem;
  font-weight: 700;
  padding: .05rem .35rem;
  border-radius: 999px;
  background: rgba(0,0,0,.06);
  color: var(--pr-text-muted);
  line-height: 1.3;
  min-width: 1.2em;
  text-align: center;
}
[data-mode="dark"] .pr-chip-count { background: rgba(255,255,255,.08); }
.pr-chip.is-active .pr-chip-count {
  background: rgba(255,255,255,.22);
  color: #fff;
}

/* Difficulty chips: subtle color hint even when inactive */
.pr-chip-diff-easy   { color: var(--pr-easy-fg); }
.pr-chip-diff-medium { color: var(--pr-medium-fg); }
.pr-chip-diff-hard   { color: var(--pr-hard-fg); }
.pr-chip-diff-easy.is-active,
.pr-chip-diff-medium.is-active,
.pr-chip-diff-hard.is-active { color: var(--pr-chip-active-fg); }

/* Hidden categories (collapsed by default; expanded class reveals) */
.pr-chip-hidden { display: none; }
.pr-chip-categories.is-expanded .pr-chip-hidden { display: inline-flex; }

/* Expand / "show fewer" toggle */
.pr-chip-expand {
  background: transparent;
  border: 1px dashed var(--pr-chip-border);
  color: var(--pr-c-blue);
  font-weight: 600;
}
.pr-chip-expand:hover {
  background: rgba(59,130,246,.06);
  border-style: solid;
}
.pr-chip-expand .pr-expand-hide { display: none; }
.pr-chip-categories.is-expanded .pr-chip-expand .pr-expand-show { display: none; }
.pr-chip-categories.is-expanded .pr-chip-expand .pr-expand-hide { display: inline; }

.pr-reset {
  margin-left: auto;
  background: transparent;
  border: 0;
  color: var(--pr-c-blue);
  font-size: .8rem;
  font-weight: 600;
  cursor: pointer;
  padding: .25rem .5rem;
  border-radius: 6px;
  white-space: nowrap;
}
.pr-reset:hover { background: rgba(59,130,246,.08); }

.pr-meta-bar {
  font-size: .85rem;
  color: var(--pr-text-muted);
  margin: 0 .25rem 1rem;
  display: flex;
  flex-wrap: wrap;
  gap: .5rem;
  align-items: center;
}
.pr-meta-bar #pr-count { font-weight: 700; color: var(--heading-color, inherit); }
.pr-active-filters { color: var(--pr-c-violet); font-weight: 500; }

/* ════════════════════════════════════════════════
   3. PROBLEM GRID
════════════════════════════════════════════════ */
.pr-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: 1rem;
  margin: 0 0 2.5rem;
}

.pr-card {
  position: relative;
  display: flex;
  flex-direction: column;
  padding: 1.1rem 1.15rem 1rem;
  background: var(--pr-card-bg);
  border: 1px solid var(--pr-border);
  border-radius: 14px;
  text-decoration: none !important;
  color: inherit;
  transition: transform .18s ease, box-shadow .18s ease, border-color .18s ease;
  box-shadow: var(--pr-card-shadow);
  overflow: hidden;
}
.pr-card::before {
  content: "";
  position: absolute; left: 0; top: 0; bottom: 0;
  width: 3px;
  background: linear-gradient(180deg, var(--pr-c-blue), var(--pr-c-violet));
  opacity: 0;
  transition: opacity .18s ease;
}
.pr-card:hover {
  transform: translateY(-3px);
  box-shadow: var(--pr-card-shadow-hover);
  border-color: rgba(59,130,246,.35);
}
.pr-card:hover::before { opacity: 1; }

.pr-card-head {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: .55rem;
}
.pr-card-id {
  font-size: .8rem;
  font-weight: 700;
  color: var(--pr-text-muted);
  font-variant-numeric: tabular-nums;
}
.pr-card-title {
  font-size: 1.02rem;
  font-weight: 700;
  line-height: 1.35;
  margin: 0 0 .55rem;
  color: var(--heading-color, inherit);
}
.pr-card-cat {
  font-size: .78rem;
  font-weight: 600;
  color: var(--pr-c-blue);
  margin-bottom: .65rem;
  letter-spacing: .01em;
}
.pr-card-topics {
  display: flex;
  flex-wrap: wrap;
  gap: .3rem;
  margin-top: auto;
  padding-top: .35rem;
}
.pr-topic {
  font-size: .7rem;
  font-weight: 500;
  padding: .2rem .55rem;
  background: rgba(0,0,0,.04);
  border-radius: 6px;
  color: var(--pr-text-muted);
  white-space: nowrap;
}
[data-mode="dark"] .pr-topic { background: rgba(255,255,255,.05); }
.pr-topic-more {
  background: rgba(59,130,246,.12);
  color: var(--pr-c-blue);
}

/* ════════════════════════════════════════════════
   4. DIFFICULTY PILLS
════════════════════════════════════════════════ */
.pr-diff {
  display: inline-flex;
  align-items: center;
  padding: .2rem .55rem;
  font-size: .7rem;
  font-weight: 700;
  letter-spacing: .04em;
  text-transform: uppercase;
  border-radius: 999px;
}
.pr-diff-easy   { background: var(--pr-easy-bg);   color: var(--pr-easy-fg); }
.pr-diff-medium { background: var(--pr-medium-bg); color: var(--pr-medium-fg); }
.pr-diff-hard   { background: var(--pr-hard-bg);   color: var(--pr-hard-fg); }

/* ════════════════════════════════════════════════
   5. EMPTY STATE
════════════════════════════════════════════════ */
.pr-empty {
  text-align: center;
  padding: 3rem 1rem;
  background: var(--pr-card-bg);
  border: 1px dashed var(--pr-border);
  border-radius: 14px;
  margin-bottom: 2rem;
}
.pr-empty-icon {
  width: 56px;
  height: 56px;
  margin: 0 auto 1rem;
  background: rgba(59,130,246,.10);
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
}
.pr-empty-icon svg { width: 24px; height: 24px; color: var(--pr-c-blue); }
.pr-empty h3 { margin: 0 0 .35rem; font-size: 1.1rem; }
.pr-empty p { margin: 0 0 1rem; color: var(--pr-text-muted); }

/* ════════════════════════════════════════════════
   6. BUTTONS
════════════════════════════════════════════════ */
.pr-btn {
  display: inline-flex;
  align-items: center;
  gap: .55rem;
  padding: .7rem 1.3rem;
  font-family: inherit;
  font-size: .95rem;
  font-weight: 600;
  border: 0;
  border-radius: 10px;
  cursor: pointer;
  text-decoration: none !important;
  transition: transform .15s ease, box-shadow .15s ease, background .15s ease;
}
.pr-btn-primary {
  background: linear-gradient(135deg, #3b82f6 0%, #8b5cf6 100%);
  color: #fff;
  box-shadow: 0 4px 12px rgba(59,130,246,.3);
}
.pr-btn-primary:hover {
  transform: translateY(-2px);
  box-shadow: 0 8px 20px rgba(59,130,246,.4);
}
.pr-btn-primary svg { width: 18px; height: 18px; }
.pr-btn-ghost {
  background: transparent;
  border: 1px solid var(--pr-border);
  color: var(--pr-text-muted);
  padding: .4rem .85rem;
  font-size: .85rem;
}
.pr-btn-ghost:hover { background: rgba(59,130,246,.08); color: var(--pr-c-blue); }

/* ════════════════════════════════════════════════
   7. PROBLEM PAGE
════════════════════════════════════════════════ */
.pr-problem { max-width: 880px; margin: 0 auto; }

.pr-breadcrumb {
  font-size: .85rem;
  color: var(--pr-text-muted);
  margin-bottom: 1.25rem;
  display: flex;
  align-items: center;
  flex-wrap: wrap;
  gap: .3rem;
}
.pr-breadcrumb a { color: var(--pr-c-blue); text-decoration: none; }
.pr-breadcrumb a:hover { text-decoration: underline; }
.pr-bc-sep { opacity: .5; }
.pr-bc-current { color: var(--heading-color, inherit); font-weight: 600; }

.pr-problem-head {
  margin-bottom: 1.5rem;
  padding-bottom: 1.5rem;
  border-bottom: 1px solid var(--pr-border);
}
.pr-problem-meta-row {
  display: flex;
  flex-wrap: wrap;
  gap: .55rem;
  align-items: center;
  margin-bottom: .85rem;
}
.pr-problem-id {
  font-size: .78rem;
  font-weight: 700;
  letter-spacing: .12em;
  text-transform: uppercase;
  color: var(--pr-text-muted);
}
.pr-cat-pill {
  font-size: .75rem;
  font-weight: 600;
  padding: .25rem .65rem;
  background: rgba(59,130,246,.1);
  color: var(--pr-c-blue);
  border-radius: 999px;
}
.pr-problem-title {
  font-size: clamp(1.6rem, 1.3rem + 1vw, 2.2rem);
  font-weight: 800;
  line-height: 1.2;
  margin: 0 0 .85rem;
  letter-spacing: -.015em;
}
.pr-problem-topics { display: flex; flex-wrap: wrap; gap: .35rem; }

.pr-question {
  font-size: 1rem;
  line-height: 1.7;
}
.pr-question h2 { margin-top: 1.8rem; }
.pr-question pre, .pr-solution-body pre {
  border-radius: 10px;
  font-size: .88rem;
}

/* Gate (Reveal button) */
.pr-solution-gate {
  margin: 2.5rem 0;
  padding: 2rem 1.5rem;
  background:
    linear-gradient(135deg, rgba(59,130,246,.06) 0%, rgba(139,92,246,.06) 100%),
    var(--pr-card-bg);
  border: 1px solid var(--pr-border);
  border-radius: 16px;
  text-align: center;
}
.pr-reveal-btn { font-size: 1rem; padding: .85rem 1.6rem; }
.pr-reveal-btn svg { width: 20px; height: 20px; }
.pr-reveal-btn.is-revealing { pointer-events: none; opacity: .8; }
.pr-gate-hint {
  margin: .9rem 0 0;
  font-size: .85rem;
  color: var(--pr-text-muted);
  font-style: italic;
}

/* Solution section after reveal */
.pr-solution {
  margin-top: 2rem;
  padding-top: 1rem;
  border-top: 2px dashed var(--pr-border);
  animation: pr-fade-in .35s ease-out;
}
@keyframes pr-fade-in {
  from { opacity: 0; transform: translateY(6px); }
  to   { opacity: 1; transform: translateY(0); }
}
.pr-solution-banner {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 1rem;
}
.pr-solution-banner h2 {
  margin: 0;
  font-size: 1.4rem;
  background: linear-gradient(135deg, var(--pr-c-blue), var(--pr-c-violet));
  -webkit-background-clip: text;
  background-clip: text;
  -webkit-text-fill-color: transparent;
}
.pr-solution-body { font-size: 1rem; line-height: 1.7; }

.pr-problem-foot {
  margin-top: 3rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--pr-border);
}
.pr-prev-next {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: .85rem;
  margin-bottom: 1.25rem;
}
.pr-nav-card {
  display: flex;
  flex-direction: column;
  padding: .85rem 1.05rem;
  background: var(--pr-card-bg);
  border: 1px solid var(--pr-border);
  border-radius: 12px;
  text-decoration: none !important;
  color: inherit;
  transition: transform .15s ease, border-color .15s ease;
}
.pr-nav-card:hover {
  transform: translateY(-2px);
  border-color: var(--pr-c-blue);
}
.pr-nav-next { text-align: right; align-items: flex-end; }
.pr-nav-dir {
  font-size: .72rem;
  font-weight: 700;
  letter-spacing: .08em;
  text-transform: uppercase;
  color: var(--pr-c-blue);
  margin-bottom: .25rem;
}
.pr-nav-title {
  font-size: .92rem;
  font-weight: 600;
  color: var(--heading-color, inherit);
}
.pr-source-link {
  display: inline-flex;
  align-items: center;
  gap: .45rem;
  font-size: .85rem;
  color: var(--pr-text-muted) !important;
  text-decoration: none !important;
}
.pr-source-link:hover { color: var(--pr-c-blue) !important; }
.pr-source-link svg { width: 16px; height: 16px; }

/* ════════════════════════════════════════════════
   8. LANDING HUB (/practice/)
════════════════════════════════════════════════ */
.pr-tracks {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: 1.25rem;
  margin: 1.5rem 0;
}
.pr-track-card {
  display: flex;
  flex-direction: column;
  padding: 1.6rem 1.4rem;
  background: var(--pr-card-bg);
  border: 1px solid var(--pr-border);
  border-radius: 16px;
  text-decoration: none !important;
  color: inherit;
  transition: transform .18s ease, box-shadow .18s ease, border-color .18s ease;
  position: relative;
  overflow: hidden;
}
.pr-track-card::after {
  content: "";
  position: absolute;
  top: -40px; right: -40px;
  width: 140px; height: 140px;
  border-radius: 50%;
  background: linear-gradient(135deg, rgba(59,130,246,.15), rgba(139,92,246,.15));
  filter: blur(20px);
  opacity: 0;
  transition: opacity .25s ease;
}
.pr-track-card:hover {
  transform: translateY(-4px);
  box-shadow: var(--pr-card-shadow-hover);
  border-color: rgba(59,130,246,.4);
}
.pr-track-card:hover::after { opacity: 1; }
.pr-track-emoji { font-size: 2rem; margin-bottom: .6rem; }
.pr-track-name {
  font-size: 1.15rem;
  font-weight: 700;
  margin: 0 0 .35rem;
  color: var(--heading-color, inherit);
}
.pr-track-desc {
  font-size: .9rem;
  color: var(--pr-text-muted);
  line-height: 1.5;
  margin: 0 0 1rem;
}
.pr-track-meta {
  display: flex;
  gap: 1rem;
  margin-top: auto;
  padding-top: .8rem;
  border-top: 1px solid var(--pr-border);
  font-size: .8rem;
  color: var(--pr-text-muted);
}
.pr-track-meta strong { color: var(--heading-color, inherit); }

.pr-track-chips {
  display: flex;
  flex-wrap: wrap;
  gap: .4rem;
  margin-top: .85rem;
  padding-top: .85rem;
  border-top: 1px dashed var(--pr-border);
}
.pr-track-chip {
  display: inline-flex;
  align-items: center;
  gap: .35rem;
  font-size: .76rem;
  padding: .2rem .6rem;
  border-radius: 999px;
  background: var(--pr-chip-bg, rgba(120, 120, 120, .1));
  color: var(--pr-text-muted);
  border: 1px solid var(--pr-border);
}
.pr-track-chip-count {
  font-weight: 700;
  font-size: .7rem;
  color: var(--heading-color, inherit);
  opacity: .75;
}
.pr-track-chip-more {
  font-style: italic;
  opacity: .8;
}
.pr-track-status {
  display: inline-block;
  font-size: .68rem;
  font-weight: 700;
  padding: .15rem .55rem;
  border-radius: 999px;
  text-transform: uppercase;
  letter-spacing: .08em;
}
.pr-track-status-live    { background: var(--pr-easy-bg); color: var(--pr-easy-fg); }
.pr-track-status-soon    { background: var(--pr-medium-bg); color: var(--pr-medium-fg); }

/* ════════════════════════════════════════════════
   9. RESPONSIVE
════════════════════════════════════════════════ */
@media (max-width: 640px) {
  .pr-hero { padding: 1.75rem 1.25rem 1.5rem; }
  .pr-toolbar { position: static; }
  .pr-prev-next { grid-template-columns: 1fr; }
  .pr-nav-next { text-align: left; align-items: flex-start; }
  .pr-stats { gap: .5rem; }
  .pr-stat { padding: .55rem .85rem; min-width: 78px; }
  .pr-stat strong { font-size: 1.2rem; }
}

/* ════════════════════════════════════════════════
   HOMEPAGE: Interview Prep promo card
════════════════════════════════════════════════ */
.hp-prep-card {
  display: block;
  margin: 0 0 1.25rem;
  padding: 0;
  text-decoration: none !important;
  border-radius: 14px;
  border: 1px solid var(--pr-border);
  background:
    radial-gradient(900px 280px at 0% 0%, rgba(59,130,246,.10), transparent 60%),
    radial-gradient(700px 220px at 100% 100%, rgba(139,92,246,.10), transparent 60%),
    var(--pr-card-bg);
  overflow: hidden;
  transition: transform .18s ease, box-shadow .18s ease, border-color .18s ease;
  color: inherit !important;
}
.hp-prep-card:hover {
  transform: translateY(-3px);
  box-shadow: var(--pr-card-shadow-hover);
  border-color: rgba(59,130,246,.4);
  color: inherit !important;
}
.hp-prep-inner {
  display: flex;
  align-items: center;
  gap: 1.25rem;
  padding: 1.4rem 1.5rem;
}
.hp-prep-icon {
  font-size: 2.6rem;
  line-height: 1;
  flex: 0 0 auto;
}
.hp-prep-body { flex: 1 1 auto; min-width: 0; }
.hp-prep-title {
  font-size: clamp(1.15rem, .9rem + .8vw, 1.5rem);
  font-weight: 800;
  line-height: 1.25;
  margin: .35rem 0 .55rem;
  letter-spacing: -.01em;
  background: linear-gradient(135deg, #3b82f6 0%, #8b5cf6 50%, #ec4899 100%);
  -webkit-background-clip: text;
  background-clip: text;
  -webkit-text-fill-color: transparent;
}
.hp-prep-desc {
  font-size: .95rem;
  line-height: 1.55;
  color: var(--pr-text-muted);
  margin: 0 0 .85rem;
}
.hp-prep-meta {
  display: flex;
  flex-wrap: wrap;
  gap: 1rem;
  font-size: .85rem;
  color: var(--pr-text-muted);
  align-items: center;
}
.hp-prep-meta strong {
  color: var(--heading-color, inherit);
  font-weight: 700;
}
.hp-prep-cta {
  margin-left: auto;
  font-weight: 700;
  color: var(--pr-c-blue);
}
.hp-prep-card:hover .hp-prep-cta { color: var(--pr-c-violet); }

@media (max-width: 640px) {
  .hp-prep-inner { padding: 1.1rem 1.2rem; gap: .9rem; }
  .hp-prep-icon { font-size: 2rem; }
  .hp-prep-cta { margin-left: 0; }
}

/* --- Feature card on practice-index pages -------------------------------- */
/* A single prominent card rendered above the filter toolbar when the index */
/* declares page.feature_card. Visually distinct from problem cards.        */

.pr-feature-card {
  display: block;
  margin: 1.5rem 0 2rem;
  padding: 0;
  background: var(--pr-card-bg);
  border: 1px solid var(--pr-border);
  border-radius: 16px;
  text-decoration: none !important;
  color: inherit;
  box-shadow: var(--pr-card-shadow);
  transition: transform .18s ease, box-shadow .18s ease, border-color .18s ease;
  position: relative;
  overflow: hidden;
}

.pr-feature-card::before {
  content: "";
  position: absolute;
  inset: 0;
  background: linear-gradient(135deg, rgba(59,130,246,.08), rgba(139,92,246,.08));
  opacity: 0;
  transition: opacity .25s ease;
  pointer-events: none;
}

.pr-feature-card:hover {
  transform: translateY(-2px);
  box-shadow: var(--pr-card-shadow-hover);
  border-color: rgba(59,130,246,.4);
}
.pr-feature-card:hover::before { opacity: 1; }

.pr-feature-card-inner {
  display: flex;
  align-items: center;
  gap: 1.2rem;
  padding: 1.4rem 1.6rem;
  position: relative;
  z-index: 1;
}

.pr-feature-card-icon {
  font-size: 2.4rem;
  line-height: 1;
  flex: 0 0 auto;
}

.pr-feature-card-body {
  flex: 1 1 auto;
  min-width: 0;
}

.pr-feature-card-eyebrow {
  font-size: .72rem;
  font-weight: 600;
  letter-spacing: .08em;
  text-transform: uppercase;
  color: var(--pr-accent, #3b82f6);
  margin-bottom: .25rem;
}

.pr-feature-card-title {
  font-size: 1.2rem;
  font-weight: 700;
  margin: 0 0 .35rem;
  color: var(--heading-color, inherit);
  line-height: 1.3;
}

.pr-feature-card-desc {
  font-size: .92rem;
  color: var(--pr-text-muted);
  margin: 0 0 .6rem;
  line-height: 1.5;
}

.pr-feature-card-meta {
  display: flex;
  gap: 1rem;
  flex-wrap: wrap;
  font-size: .78rem;
  color: var(--pr-text-muted);
}
.pr-feature-card-meta span {
  display: inline-flex;
  align-items: center;
}
.pr-feature-card-meta span + span::before {
  content: "•";
  margin-right: 1rem;
  opacity: .5;
}

.pr-feature-card-arrow {
  font-size: 1.5rem;
  color: var(--pr-text-muted);
  flex: 0 0 auto;
  transition: transform .2s ease, color .2s ease;
}
.pr-feature-card:hover .pr-feature-card-arrow {
  transform: translateX(4px);
  color: var(--pr-accent, #3b82f6);
}

@media (max-width: 640px) {
  .pr-feature-card-inner { padding: 1.1rem 1.2rem; gap: .9rem; }
  .pr-feature-card-icon { font-size: 2rem; }
  .pr-feature-card-arrow { display: none; }
}

/* ══════════════════════════════════════════════
   SYSTEM DESIGN CONCEPTS
   Index page + per-topic page styles.
   Reuses .pr-hero, .pr-stats, .pr-toolbar, .pr-chip from above.
══════════════════════════════════════════════ */

.concept-sections {
  display: flex;
  flex-direction: column;
  gap: 2.25rem;
  margin-top: 1.5rem;
}

.concept-section { display: block; }

.concept-section-head {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: 1rem;
  padding-bottom: .55rem;
  margin-bottom: 1rem;
  border-bottom: 1px solid var(--pr-border);
}

.concept-section-name {
  margin: 0;
  font-size: 1.15rem;
  font-weight: 700;
  letter-spacing: -.01em;
  color: var(--heading-color);
}

.concept-section-count {
  font-size: .82rem;
  color: var(--pr-text-muted);
}

.concept-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
  gap: .9rem;
}

.concept-card {
  position: relative;
  display: flex;
  flex-direction: column;
  padding: 1.05rem 1.1rem 1.15rem;
  background: var(--pr-card-bg);
  border: 1px solid var(--pr-border);
  border-radius: 12px;
  color: inherit;
  text-decoration: none;
  transition: transform .18s ease, border-color .18s ease, box-shadow .18s ease;
}

.concept-card:hover {
  transform: translateY(-2px);
  border-color: var(--pr-accent, #3b82f6);
  box-shadow: 0 6px 22px rgba(0,0,0,.08);
  text-decoration: none;
}

.concept-card-head {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: .5rem;
  margin-bottom: .55rem;
}

.concept-card-id {
  font-size: .72rem;
  font-weight: 700;
  letter-spacing: .04em;
  color: var(--pr-text-muted);
  font-variant-numeric: tabular-nums;
}

.concept-card-status {
  display: inline-block;
  font-size: .64rem;
  font-weight: 700;
  letter-spacing: .08em;
  text-transform: uppercase;
  padding: .14rem .5rem;
  border-radius: 999px;
}

.concept-card-status-live  { background: var(--pr-easy-bg);  color: var(--pr-easy-fg); }
.concept-card-status-draft { background: var(--pr-hard-bg, rgba(180, 90, 20, .12)); color: var(--pr-hard-fg, #b45a14); opacity: .85; }

.concept-card-title {
  margin: 0 0 .35rem;
  font-size: 1rem;
  font-weight: 700;
  line-height: 1.3;
  color: var(--heading-color);
  letter-spacing: -.005em;
}

.concept-card-tease {
  margin: 0;
  font-size: .87rem;
  line-height: 1.5;
  color: var(--pr-text-muted);
}

.concept-card-arrow {
  position: absolute;
  right: .85rem;
  bottom: .85rem;
  font-size: 1rem;
  color: var(--pr-text-muted);
  opacity: 0;
  transform: translateX(-4px);
  transition: opacity .18s ease, transform .18s ease, color .18s ease;
}
.concept-card:hover .concept-card-arrow {
  opacity: 1;
  transform: translateX(0);
  color: var(--pr-accent, #3b82f6);
}

/* ── Per-topic page ──────────────────────── */

.concept-breadcrumb {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: .5rem;
  font-size: .85rem;
  color: var(--pr-text-muted);
  margin-bottom: 1rem;
}

.concept-breadcrumb a {
  color: var(--pr-text-muted);
  text-decoration: none;
}
.concept-breadcrumb a:hover { color: var(--link-color); text-decoration: underline; }

.concept-breadcrumb-current { color: var(--heading-color); font-weight: 600; }

.concept-hero {
  padding: 1.5rem 0 1.75rem;
  border-bottom: 1px solid var(--pr-border);
  margin-bottom: 2rem;
}

.concept-section-pill {
  display: inline-block;
  font-size: .72rem;
  font-weight: 700;
  letter-spacing: .1em;
  text-transform: uppercase;
  color: var(--pr-accent, #3b82f6);
  padding: .25rem .65rem;
  border: 1px solid var(--pr-accent, #3b82f6);
  border-radius: 999px;
  margin-bottom: .9rem;
}

.concept-title {
  margin: 0 0 .65rem;
  font-size: 2rem;
  font-weight: 800;
  line-height: 1.2;
  letter-spacing: -.02em;
  color: var(--heading-color);
}

.concept-tease {
  margin: 0;
  font-size: 1.05rem;
  line-height: 1.55;
  color: var(--pr-text-muted);
  max-width: 60ch;
}

.concept-placeholder {
  padding: 2.25rem 1.5rem;
  border: 1px dashed var(--pr-border);
  border-radius: 14px;
  background: var(--pr-card-bg);
  text-align: center;
  margin-bottom: 2.5rem;
}

.concept-placeholder-icon {
  font-size: 2.25rem;
  margin-bottom: .65rem;
}

.concept-placeholder-title {
  margin: 0 0 .65rem;
  font-size: 1.2rem;
  font-weight: 700;
  color: var(--heading-color);
}

.concept-placeholder-body {
  margin: 0 auto 1rem;
  max-width: 52ch;
  color: var(--pr-text-muted);
  font-size: .95rem;
  line-height: 1.55;
}

.concept-placeholder-list {
  display: inline-block;
  text-align: left;
  margin: 0 auto;
  padding-left: 1.1rem;
  color: var(--pr-text-muted);
  font-size: .92rem;
  line-height: 1.7;
}
.concept-placeholder-list a { color: var(--link-color); }

.concept-body {
  max-width: 70ch;
  margin: 0 auto 2.5rem;
}

.concept-nav {
  display: grid;
  grid-template-columns: 1fr 1fr 1fr;
  gap: .65rem;
  margin-top: 2rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--pr-border);
}

.concept-nav-link {
  display: flex;
  flex-direction: column;
  gap: .15rem;
  padding: .75rem .9rem;
  border: 1px solid var(--pr-border);
  border-radius: 10px;
  background: var(--pr-card-bg);
  color: inherit;
  text-decoration: none;
  transition: border-color .18s ease, transform .18s ease;
  min-height: 60px;
}
.concept-nav-link:hover {
  border-color: var(--pr-accent, #3b82f6);
  transform: translateY(-1px);
  text-decoration: none;
}
.concept-nav-empty { visibility: hidden; }

.concept-nav-direction {
  font-size: .7rem;
  font-weight: 700;
  letter-spacing: .08em;
  text-transform: uppercase;
  color: var(--pr-text-muted);
}
.concept-nav-title {
  font-size: .9rem;
  font-weight: 600;
  color: var(--heading-color);
  line-height: 1.3;
}
.concept-nav-prev  { text-align: left; }
.concept-nav-index { text-align: center; align-items: center; }
.concept-nav-next  { text-align: right; align-items: flex-end; }

@media (max-width: 640px) {
  .concept-title { font-size: 1.55rem; }
  .concept-nav { grid-template-columns: 1fr; }
  .concept-nav-prev, .concept-nav-index, .concept-nav-next { text-align: left; align-items: stretch; }
}