/* ============================================
   Stanley's Knowledge Space — Apple Design
   ============================================ */

:root {
  --color-bg:              #ffffff;
  --color-bg-alt:          #f5f5f7;
  --color-text:            #1d1d1f;
  --color-text-secondary:  #86868b;
  --color-text-tertiary:   #a1a1a6;
  --color-link:            #06c;
  --color-link-hover:      #0077ed;
  --color-border:          #d2d2d7;
  --color-nav-bg:          rgba(251, 251, 253, 0.92);
  --color-nav-border:      rgba(0, 0, 0, 0.08);
  --dark-bg:               #000000;
  --dark-text:             #f5f5f7;
  --dark-text-secondary:   #a1a1a6;
  --dark-link:             #2997ff;
  --dark-link-hover:       #52abff;
  --font-sans:  -apple-system, BlinkMacSystemFont, 'SF Pro Display', 'SF Pro Text', 'Helvetica Neue', 'PingFang SC', 'Noto Sans SC', sans-serif;
  --font-mono:  'SF Mono', SFMono-Regular, Menlo, Monaco, Consolas, monospace;
  --ease-out:    cubic-bezier(0.25, 0.46, 0.45, 0.94);
  --ease-spring: cubic-bezier(0.16, 1, 0.3, 1);
  --max-width: 980px;
}

*, *::before, *::after { margin: 0; padding: 0; box-sizing: border-box; }
html { scroll-behavior: smooth; -webkit-font-smoothing: antialiased; -moz-osx-font-smoothing: grayscale; text-rendering: optimizeLegibility; }
body { font-family: var(--font-sans); background: var(--color-bg); color: var(--color-text); line-height: 1.47; overflow-x: hidden; }
a { text-decoration: none; color: inherit; }
button { border: none; background: none; cursor: pointer; font-family: inherit; }
code { font-family: var(--font-mono); }

/* --- Navigation --- */
.nav {
  position: fixed; top: 0; left: 0; right: 0; z-index: 9990;
  background: var(--color-nav-bg);
  backdrop-filter: saturate(180%) blur(20px);
  -webkit-backdrop-filter: saturate(180%) blur(20px);
  border-bottom: 0.5px solid var(--color-nav-border);
}
.nav-inner {
  max-width: 1024px; margin: 0 auto; padding: 0 22px; height: 48px;
  display: flex; align-items: center; justify-content: space-between;
}
.nav-logo { font-size: 1.05rem; font-weight: 600; letter-spacing: -0.02em; color: var(--color-text); transition: opacity 0.3s var(--ease-out); }
.nav-logo:hover { opacity: 0.6; }
.nav-links { display: flex; gap: 28px; align-items: center; }
.nav-link {
  font-size: 0.72rem; font-weight: 400; color: var(--color-text-secondary); letter-spacing: 0;
  transition: color 0.3s var(--ease-out); display: inline-flex; align-items: center;
}
.nav-link:hover { color: var(--color-text); }

/* --- Hero --- */
.hero {
  padding-top: 48px; min-height: 92vh;
  display: flex; align-items: center; justify-content: center; text-align: center;
  background: var(--color-bg);
}
.hero-content { max-width: 720px; padding: 0 24px; }
.hero-eyebrow {
  font-size: 1rem; font-weight: 500; color: var(--color-text-secondary);
  letter-spacing: 0; margin-bottom: 12px;
}
.hero-title {
  font-size: clamp(3.2rem, 9vw, 5.6rem); font-weight: 700;
  letter-spacing: -0.04em; line-height: 1.04; margin-bottom: 18px; color: var(--color-text);
}
.hero-subtitle {
  font-size: clamp(1.2rem, 2.8vw, 1.55rem); font-weight: 400;
  color: var(--color-text-secondary); line-height: 1.45; letter-spacing: -0.01em;
  max-width: 520px; margin: 0 auto 36px;
}
.hero-links { display: flex; gap: 32px; justify-content: center; }
.hero-link {
  font-size: 1.2rem; font-weight: 400; display: inline-flex; align-items: center; gap: 6px;
  transition: gap 0.3s var(--ease-out);
}
.hero-link--blue { color: var(--color-link); }
.hero-link--blue:hover { color: var(--color-link-hover); gap: 10px; }
.hero-link--secondary { color: var(--color-text-secondary); }
.hero-link--secondary:hover { color: var(--color-text); gap: 10px; }
.hero-link svg { width: 11px; height: 11px; transition: transform 0.3s var(--ease-out); }
.hero-link:hover svg { transform: translateX(2px); }

/* --- Scroll Reveal --- */
.reveal { opacity: 0; transform: translateY(24px); transition: opacity 0.8s var(--ease-spring), transform 0.8s var(--ease-spring); }
.reveal.visible { opacity: 1; transform: translateY(0); }
.reveal-delay-1 { transition-delay: 0.1s; }
.reveal-delay-2 { transition-delay: 0.2s; }
.reveal-delay-3 { transition-delay: 0.35s; }

/* --- Filter --- */
.filter-section { padding: 56px 24px 0; background: var(--color-bg); text-align: center; }
.filter-section .section-label { font-size: 0.8rem; font-weight: 600; text-transform: uppercase; letter-spacing: 0.06em; color: var(--color-text-tertiary); margin-bottom: 16px; }
.filter-bar { display: flex; gap: 32px; justify-content: center; flex-wrap: wrap; }
.filter-link { font-size: 0.92rem; font-weight: 400; color: var(--color-link); padding-bottom: 2px; border-bottom: 1px solid transparent; transition: border-color 0.3s var(--ease-out); }
.filter-link:hover { border-bottom-color: var(--color-link); }
.filter-link.active { color: var(--color-text); border-bottom-color: transparent; cursor: default; }

/* --- Billboards --- */
.billboards { background: var(--color-bg); }
.billboard {
  position: relative; display: block; text-align: center; padding: 80px 24px;
  margin-bottom: 12px; overflow: hidden;
  transition: opacity 0.4s var(--ease-out), transform 0.4s var(--ease-out);
}
.billboard:last-child { margin-bottom: 0; }
.billboard--light { background: var(--color-bg-alt); color: var(--color-text); }
.billboard--light .billboard-link { color: var(--color-link); }
.billboard--light .billboard-link:hover { color: var(--color-link-hover); }
.billboard--dark { background: var(--dark-bg); color: var(--dark-text); }
.billboard--dark .billboard-tagline { color: var(--dark-text-secondary); }
.billboard--dark .billboard-description { color: var(--dark-text-secondary); }
.billboard--dark .billboard-link { color: var(--dark-link); }
.billboard--dark .billboard-link:hover { color: var(--dark-link-hover); }
.billboard--dark .billboard-date { color: #6e6e73; }
.billboard--tinted { background: linear-gradient(180deg, #fbfbfd 0%, #f0f0f5 100%); color: var(--color-text); }
.billboard--tinted .billboard-link { color: var(--color-link); }
.billboard--tinted .billboard-link:hover { color: var(--color-link-hover); }

.billboard-inner { max-width: var(--max-width); margin: 0 auto; }
.billboard-category {
  font-size: 0.75rem; font-weight: 500; text-transform: uppercase; letter-spacing: 0.06em;
  color: var(--color-text-tertiary); margin-bottom: 12px;
}
.billboard--dark .billboard-category { color: #6e6e73; }
.billboard-title {
  font-size: clamp(2.2rem, 5.5vw, 3.6rem); font-weight: 700;
  letter-spacing: -0.035em; line-height: 1.08; margin-bottom: 8px;
}
.billboard-tagline {
  font-size: clamp(1.05rem, 2.2vw, 1.3rem); font-weight: 400;
  color: var(--color-text-secondary); line-height: 1.4; margin-bottom: 6px; letter-spacing: -0.01em;
}
.billboard-description {
  font-size: 0.95rem; color: var(--color-text-secondary); line-height: 1.5;
  max-width: 480px; margin: 0 auto 20px;
}
.billboard-links { display: flex; gap: 28px; justify-content: center; }
.billboard-link {
  font-size: 1.1rem; font-weight: 400; display: inline-flex; align-items: center; gap: 5px;
  transition: gap 0.25s var(--ease-out);
}
.billboard-link:hover { gap: 9px; }
.billboard-link svg { width: 10px; height: 10px; transition: transform 0.25s var(--ease-out); }
.billboard-link:hover svg { transform: translateX(1px); }
.billboard-date { font-size: 0.75rem; color: var(--color-text-tertiary); margin-top: 20px; letter-spacing: 0; }
.billboard.hidden { display: none; }

/* External link indicator */
.billboard-link--external svg { width: 11px; height: 11px; }

/* --- Empty State --- */
.empty-state { text-align: center; padding: 100px 24px; background: var(--color-bg); }
.empty-title { font-size: 1.4rem; font-weight: 600; letter-spacing: -0.02em; margin-bottom: 8px; }
.empty-desc { font-size: 1rem; color: var(--color-text-secondary); max-width: 420px; margin: 0 auto; line-height: 1.5; }
.empty-desc code { font-size: 0.9em; background: var(--color-bg-alt); padding: 1px 6px; border-radius: 4px; }

/* --- About --- */
.about { padding: 100px 24px; background: var(--color-bg-alt); }
.about-inner { max-width: var(--max-width); margin: 0 auto; display: grid; grid-template-columns: 1fr 1fr; gap: 64px; align-items: center; }
.about-eyebrow { font-size: 0.8rem; font-weight: 600; text-transform: uppercase; letter-spacing: 0.06em; color: var(--color-text-tertiary); margin-bottom: 12px; }
.about-heading { font-size: clamp(1.8rem, 4vw, 2.8rem); font-weight: 700; letter-spacing: -0.035em; line-height: 1.08; margin-bottom: 20px; }
.about-text { font-size: 1rem; color: var(--color-text-secondary); line-height: 1.65; margin-bottom: 24px; }
.about-text strong { color: var(--color-text); font-weight: 600; }

/* Tags */
.about-tags { display: flex; flex-wrap: wrap; gap: 8px; }
.tag {
  font-size: 0.75rem; font-weight: 500; padding: 6px 14px;
  background: var(--color-bg); border-radius: 980px;
  border: 0.5px solid rgba(0,0,0,0.08); color: var(--color-text-secondary);
  letter-spacing: -0.01em;
}

/* Stats grid */
.about-stats { display: grid; grid-template-columns: 1fr 1fr; gap: 14px; }
.stat-card {
  padding: 24px 20px; background: var(--color-bg); border-radius: 14px;
  border: 0.5px solid rgba(0,0,0,0.06);
}
.stat-number { font-size: 1.3rem; font-weight: 700; letter-spacing: -0.02em; margin-bottom: 4px; }
.stat-label { font-size: 0.78rem; color: var(--color-text-secondary); line-height: 1.35; }

/* --- Footer --- */
.footer { padding: 36px 24px; border-top: 0.5px solid var(--color-border); background: var(--color-bg); }
.footer-inner { max-width: var(--max-width); margin: 0 auto; display: flex; justify-content: space-between; align-items: center; }
.footer-copy { font-size: 0.75rem; color: var(--color-text-tertiary); }
.footer-links { display: flex; gap: 20px; }
.footer-link {
  display: inline-flex; align-items: center; gap: 5px;
  font-size: 0.75rem; color: var(--color-text-secondary); transition: color 0.2s var(--ease-out);
}
.footer-link:hover { color: var(--color-text); }
.footer-link svg { width: 16px; height: 16px; }

/* --- Responsive --- */
@media (max-width: 768px) {
  .nav-links { gap: 18px; }
  .nav-link { font-size: 0.7rem; }
  .hero { min-height: 80vh; }
  .hero-title { margin-bottom: 14px; }
  .hero-links { gap: 24px; }
  .hero-link { font-size: 1.05rem; }
  .billboard { padding: 64px 24px; }
  .about-inner { grid-template-columns: 1fr; gap: 40px; }
  .footer-inner { flex-direction: column; gap: 12px; text-align: center; }
}
@media (max-width: 480px) {
  .hero-title { font-size: 2.6rem; }
  .hero-subtitle { font-size: 1.05rem; }
  .hero-links { flex-direction: column; gap: 14px; align-items: center; }
  .billboard-title { font-size: 1.8rem; }
  .filter-bar { gap: 18px; }
  .about-stats { grid-template-columns: 1fr; }
}