/* ============================================================
   CSS VARIABLES — one place to control the whole color system
   ============================================================ */
:root {
  /* backgrounds */
  --bg:           #F7F3EC;   /* warm cream page background */
  --bg-card:      #FFFFFF;   /* white cards */
  --bg-dark:      #1C1C1E;   /* dark sections / navbar */

  /* text */
  --text-primary:   #1A1A1A;
  --text-secondary: #5C5C5C;
  --text-on-dark:   #F7F3EC;

  /* border */
  --border:       #E0D9CF;

  /* accent (active pills, links) */
  --accent:       #D8C8F0;   /* soft lavender, like the screenshot */
  --accent-text:  #3D1F7A;

  /* algorithm visualizer colors (from project spec) */
  --viz-frontier: #3B8BD4;   /* blue  — frontier / candidates */
  --viz-active:   #EF9F27;   /* amber — currently expanding */
  --viz-visited:  #63992A;   /* green — visited / settled */
  --viz-goal:     #E24B4A;   /* red   — goal found */
  --viz-inactive: #8F8F8F;   /* gray  — unvisited */
  --viz-weight:   #9B59B6;   /* purple — weights / activations */
  --viz-boundary: #1ABC9C;   /* teal  — decision boundaries */

  /* spacing & shape */
  --radius-sm:   8px;
  --radius-md:   16px;
  --radius-pill: 999px;
  --max-width:   1100px;
}

/* ============================================================
   RESET — strip browser default margin/padding/box model
   ============================================================ */
*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

/* ============================================================
   BASE — body font, background, text color
   ============================================================ */
body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  background-color: var(--bg);
  color: var(--text-primary);
  line-height: 1.6;
}

/* ============================================================
   CONTAINER — centers content and limits max width
   ============================================================ */
.container {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 2rem 1.5rem;
}

/* ============================================================
   TYPOGRAPHY
   ============================================================ */
h1 { font-size: 2.2rem; font-weight: 700; letter-spacing: -0.5px; }
h2 { font-size: 1.5rem; font-weight: 600; }
h3 { font-size: 1.1rem; font-weight: 600; }

p  { color: var(--text-secondary); }

/* ============================================================
   PILL TAGS  (Beginner / Intermediate / Advanced badges)
   ============================================================ */
.badge {
  display: inline-block;
  padding: 0.25rem 0.85rem;
  border-radius: var(--radius-pill);
  font-size: 0.75rem;
  font-weight: 600;
  letter-spacing: 0.4px;
  border: 1.5px solid currentColor;
}

.badge-beginner     { color: #2E7D32; background: #E8F5E9; }
.badge-intermediate { color: #E65100; background: #FFF3E0; }
.badge-advanced     { color: #6A1B9A; background: #F3E5F5; }

/* ============================================================
   BUTTONS
   ============================================================ */
.btn {
  display: inline-flex;
  align-items: center;
  gap: 0.4rem;
  padding: 0.5rem 1.2rem;
  border-radius: var(--radius-pill);
  border: 1.5px solid var(--border);
  background: var(--bg-card);
  color: var(--text-primary);
  font-size: 0.9rem;
  font-weight: 500;
  cursor: pointer;
  transition: background 0.15s, border-color 0.15s;
}

.btn:hover {
  background: var(--accent);
  border-color: var(--accent-text);
  color: var(--accent-text);
}

.btn-primary {
  background: var(--bg-dark);
  color: var(--text-on-dark);
  border-color: var(--bg-dark);
}

.btn-primary:hover {
  background: #333;
  border-color: #333;
  color: var(--text-on-dark);
}

/* ============================================================
   NAVBAR
   ============================================================ */
.navbar {
  background: var(--bg-card);          /* white */
  border-bottom: 1.5px solid var(--border);
  position: sticky;
  top: 0;
  z-index: 100;
}

.nav-inner {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 0 1.5rem;
  height: 56px;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.nav-logo {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  text-decoration: none;
}

.nav-logo-text {
  font-weight: 700;
  font-size: 0.95rem;
  color: var(--text-primary);
  letter-spacing: -0.3px;
}

.nav-logo-accent {
  color: #7C3AED;   /* purple accent on "Notes" */
}

.nav-links {
  display: flex;
  gap: 0.25rem;
}

.nav-link {
  padding: 0.35rem 0.9rem;
  border-radius: var(--radius-pill);
  color: var(--text-secondary);
  text-decoration: none;
  font-size: 0.9rem;
  font-weight: 500;
  transition: background 0.15s, color 0.15s;
}

.nav-link:hover,
.nav-link--active {
  background: var(--bg);
  color: var(--text-primary);
}

/* ============================================================
   HOMEPAGE — hero, filter bar, card grid
   ============================================================ */
.hero {
  padding: 3rem 0 1.5rem;
}

.hero h1 {
  margin-bottom: 0.5rem;
}

.filter-bar {
  display: flex;
  gap: 0.5rem;
  margin-bottom: 2rem;
}

.filter-btn.active {
  background: var(--accent);
  border-color: var(--accent-text);
  color: var(--accent-text);
}

/* Grid: 3 columns on desktop, fewer on smaller screens */
.card-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
  gap: 1.25rem;
}

/* ── Card ── */
.card {
  background: var(--bg-card);
  border: 1.5px solid var(--border);
  border-radius: var(--radius-md);
  padding: 1.5rem;
  text-decoration: none;
  color: var(--text-primary);
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
  transition: border-color 0.2s, box-shadow 0.2s, transform 0.2s;
}

.card:hover {
  border-color: #C4B5FD;
  box-shadow: 0 8px 24px rgba(124,58,237,0.08);
  transform: translateY(-2px);
}

/* Locked cards — dimmed, not clickable */
.card--locked {
  opacity: 0.45;
  cursor: default;
  pointer-events: none;
}

.card-top {
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.card-topic {
  font-size: 0.7rem;
  font-weight: 700;
  color: var(--text-secondary);
  text-transform: uppercase;
  letter-spacing: 0.7px;
}

.card-title {
  font-size: 0.95rem;
  font-weight: 700;
  line-height: 1.4;
}

.card-desc {
  font-size: 0.875rem;
  line-height: 1.55;
  flex: 1;
}

/* Bottom CTA row */
.card-cta {
  margin-top: 0.25rem;
  display: flex;
  justify-content: flex-end;
}

.card-cta-link {
  font-size: 0.82rem;
  font-weight: 600;
  color: #7C3AED;
}

.card-cta-soon {
  font-size: 0.82rem;
  color: var(--text-secondary);
}

.card-status {
  color: var(--text-secondary);
}

.card-status.done {
  color: #2E7D32;
  font-weight: 600;
}

/* ============================================================
   MOBILE — shared (applies to all pages)
   ============================================================ */
@media (max-width: 640px) {

  /* Smaller container padding on narrow screens */
  .container {
    padding: 1rem;
  }

  /* Hero */
  .hero {
    padding: 1.5rem 0 1rem;
  }

  h1 { font-size: 1.6rem; }

  /* Cards fill full width, single column */
  .card-grid {
    grid-template-columns: 1fr;
  }

  /* Filter bar scrolls horizontally if needed */
  .filter-bar {
    overflow-x: auto;
    padding-bottom: 0.25rem;
    flex-wrap: nowrap;
  }
}