/* ============================================================
   Design tokens
   ============================================================ */
:root {
  --color-bg:             #f7f6f3;
  --color-surface:        #ffffff;
  --color-border:         #e2dfd9;
  --color-border-light:   #eeebe5;

  --color-text-primary:   #1a1a2e;
  --color-text-secondary: #5a5a7a;
  /* 4.78:1 on the page background (WCAG AA needs 4.5:1) — it carries the
     per-control teaching notes in .control-hint. */
  --color-text-muted:     #6b6b85;

  --color-accent:         #2563eb;
  --color-accent-hover:   #1d4ed8;
  --color-accent-light:   #dbeafe;

  --font-body:   'Inter', 'Segoe UI', system-ui, sans-serif;
  --font-mono:   'JetBrains Mono', 'Fira Code', 'Consolas', monospace;

  --size-base:   15px;
  --lh-body:     1.65;

  --radius-sm:   4px;
  --radius-md:   8px;
  --radius-lg:   12px;

  --shadow-sm:   0 1px 2px rgba(0,0,0,0.06);
  --shadow-md:   0 2px 8px rgba(0,0,0,0.08), 0 1px 2px rgba(0,0,0,0.04);
  --shadow-lg:   0 4px 20px rgba(0,0,0,0.10), 0 1px 4px rgba(0,0,0,0.06);

  --nav-height:  56px;
}

/* ============================================================
   Reset / base
   ============================================================ */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

html {
  font-size: var(--size-base);
  scroll-behavior: smooth;
  height: 100dvh;
}

body {
  font-family: var(--font-body);
  background: var(--color-bg);
  color: var(--color-text-primary);
  line-height: var(--lh-body);
  height: 100dvh;
  display: flex;
  flex-direction: column;
}

main {
  flex: 1 1 0;
  display: flex;
  flex-direction: column;
  min-height: 0;
}

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

/* ============================================================
   Navigation
   ============================================================ */
.site-nav {
  position: sticky;
  top: 0;
  z-index: 100;
  height: var(--nav-height);
  background: var(--color-surface);
  border-bottom: 1px solid var(--color-border);
  box-shadow: var(--shadow-sm);
}

.nav-inner {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 24px;
  height: 100%;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 24px;
}

.nav-brand {
  font-weight: 700;
  font-size: 1rem;
  letter-spacing: -0.01em;
  color: var(--color-text-primary);
  white-space: nowrap;
}

.nav-brand:hover { color: var(--color-accent); }

/* Dropdown */
.nav-dropdown { position: relative; }

.nav-dropdown-trigger {
  background: none;
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  padding: 6px 12px;
  font-family: var(--font-body);
  font-size: 0.9rem;
  font-weight: 500;
  color: var(--color-text-primary);
  cursor: pointer;
  transition: border-color 0.15s, background 0.15s;
  white-space: nowrap;
}

.nav-dropdown-trigger:hover {
  border-color: var(--color-accent);
  background: var(--color-accent-light);
}

.nav-dropdown-menu {
  display: none;
  position: absolute;
  right: 0;
  top: calc(100% + 6px);
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  box-shadow: var(--shadow-lg);
  min-width: 280px;
  list-style: none;
  padding: 6px 0;
  max-height: 70vh;
  overflow-y: auto;
}

.nav-dropdown-menu.open { display: block; }

.nav-dropdown-menu li a {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 8px 16px;
  font-size: 0.875rem;
  color: var(--color-text-primary);
  transition: background 0.1s;
}

.nav-dropdown-menu li a:hover {
  background: var(--color-accent-light);
  color: var(--color-accent);
}

.dropdown-num {
  font-family: var(--font-mono);
  font-size: 0.75rem;
  color: var(--color-text-muted);
  min-width: 22px;
}

/* ============================================================
   Footer
   ============================================================ */
.site-footer {
  border-top: 1px solid var(--color-border);
  background: var(--color-surface);
  padding: 16px 24px;
}

.footer-inner {
  max-width: 1200px;
  margin: 0 auto;
  display: flex;
  justify-content: space-between;
  align-items: center;
  font-size: 0.8rem;
  color: var(--color-text-muted);
}

/* ============================================================
   Lecture placeholder page
   ============================================================ */
.lecture-placeholder {
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: calc(100dvh - var(--nav-height) - 56px);
  padding: 48px 24px;
}

.placeholder-inner {
  max-width: 600px;
  text-align: center;
}

.placeholder-num {
  display: inline-block;
  font-family: var(--font-mono);
  font-size: 0.8rem;
  font-weight: 500;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--color-accent);
  background: var(--color-accent-light);
  border-radius: 99px;
  padding: 4px 12px;
  margin-bottom: 16px;
}

.placeholder-inner h1 {
  font-size: 2rem;
  font-weight: 700;
  letter-spacing: -0.02em;
  line-height: 1.2;
  margin-bottom: 12px;
}

.placeholder-desc {
  color: var(--color-text-secondary);
  font-size: 1rem;
  margin-bottom: 10px;
}

.placeholder-coming {
  font-size: 0.875rem;
  color: var(--color-text-muted);
  margin-bottom: 40px;
}

.placeholder-nav {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 12px;
  flex-wrap: wrap;
}

.btn-nav {
  display: inline-block;
  padding: 8px 18px;
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  font-size: 0.875rem;
  font-weight: 500;
  color: var(--color-text-primary);
  background: var(--color-surface);
  transition: border-color 0.15s, background 0.15s, color 0.15s;
}

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

.btn-home {
  background: var(--color-accent);
  border-color: var(--color-accent);
  color: #fff;
}

.btn-home:hover {
  background: var(--color-accent-hover);
  border-color: var(--color-accent-hover);
  color: #fff;
}

/* ============================================================
   Utilities
   ============================================================ */
/* Same 600px phone breakpoint as landing.css. */
@media (max-width: 600px) {
  .hide-mobile { display: none; }

  /* A narrower gutter, so "Lectures ▾" fits beside the brand on a 320px phone. */
  .nav-inner { padding: 0 16px; gap: 12px; }
}