/* ═══════════════════════════════════════════════════════════
   Calculus Lab — derivatives, integrals & limits
   navy · paper white · millimeter grid · editorial sans
   ═══════════════════════════════════════════════════════════ */

:root {
  --navy: #12243B;
  --navy-2: #1A3352;
  --navy-deep: #0C1826;
  --paper: #FAF7F0;
  --paper-2: #F0EBDE;
  --ink: #1A1A18;
  --ink-soft: #4C4A44;
  --muted: #8A8578;
  --line: #D9D2C0;
  --deriv: #E8742E;      /* derivative orange */
  --integral: #2E8B57;   /* integral green */
  --limit: #3B6EA8;      /* limit blue */
  --grid-major: rgba(26, 26, 24, 0.10);
  --grid-minor: rgba(26, 26, 24, 0.045);

  --font-display: 'Space Grotesk', 'Helvetica Neue', sans-serif;
  --font-serif: 'Source Serif 4', Georgia, serif;
  --font-mono: 'JetBrains Mono', monospace;
  --pad: clamp(1.2rem, 4vw, 3rem);
}

* { margin: 0; padding: 0; box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  font-family: var(--font-serif);
  font-size: 18px;
  line-height: 1.7;
  color: var(--ink);
  background: var(--paper);
  -webkit-font-smoothing: antialiased;
  overflow-x: hidden;
}
.mono { font-family: var(--font-mono); }
::selection { background: var(--deriv); color: var(--paper); }
a { color: inherit; }
img { max-width: 100%; display: block; }
:focus-visible { outline: 2px solid var(--deriv); outline-offset: 3px; }

/* millimeter grid background */
body::before {
  content: '';
  position: fixed; inset: 0; z-index: 0; pointer-events: none;
  background-image:
    linear-gradient(to right, var(--grid-minor) 1px, transparent 1px),
    linear-gradient(to bottom, var(--grid-minor) 1px, transparent 1px),
    linear-gradient(to right, var(--grid-major) 1px, transparent 1px),
    linear-gradient(to bottom, var(--grid-major) 1px, transparent 1px);
  background-size: 12px 12px, 12px 12px, 60px 60px, 60px 60px;
}
.progress {
  position: fixed; top: 0; left: 0; height: 3px; width: 100%; z-index: 300;
  background: linear-gradient(90deg, var(--deriv), var(--integral));
  transform: scaleX(0); transform-origin: left;
}

/* ═══════════ NAV ═══════════ */
.site-nav {
  position: sticky; top: 0; z-index: 200;
  background: rgba(250, 247, 240, 0.92);
  backdrop-filter: blur(8px);
  border-bottom: 1px solid var(--line);
}
.nav-inner {
  max-width: 1120px; margin: 0 auto; padding: 0.85rem var(--pad);
  display: flex; align-items: baseline; justify-content: space-between; gap: 1rem;
}
.logo {
  font-family: var(--font-display); font-weight: 700; font-size: 1.25rem;
  text-decoration: none; color: var(--ink);
}
.logo-lab { color: var(--deriv); }
.nav-links { display: flex; gap: clamp(0.8rem, 2.4vw, 1.8rem); flex-wrap: wrap; }
.nav-links a {
  font-family: var(--font-mono); font-size: 0.74rem; letter-spacing: 0.14em;
  text-transform: uppercase; color: var(--ink-soft); text-decoration: none;
  padding-bottom: 0.2rem; border-bottom: 2px solid transparent;
  transition: color 0.25s ease, border-color 0.25s ease;
}
.nav-links a:hover { color: var(--ink); border-bottom-color: var(--deriv); }

/* ═══════════ HERO ═══════════ */
.hero {
  max-width: 1120px; margin: 0 auto;
  padding: clamp(3rem, 8vh, 5rem) var(--pad) 2.5rem;
  display: grid; grid-template-columns: 1.05fr 0.95fr; gap: clamp(1.6rem, 4vw, 3rem);
  align-items: center;
}
.kicker {
  font-family: var(--font-mono); font-size: 0.74rem; letter-spacing: 0.22em;
  text-transform: uppercase; color: var(--deriv); margin-bottom: 1rem;
}
.hero-title {
  font-family: var(--font-display); font-weight: 700;
  font-size: clamp(2.4rem, 6vw, 4.2rem); line-height: 1.02; letter-spacing: -0.02em;
}
.hero-title em { font-style: italic; color: var(--deriv); font-weight: 400; }
.hero-sub { margin-top: 1.2rem; font-size: 1.08rem; color: var(--ink-soft); max-width: 44ch; }
.hero-ctrl {
  margin-top: 1.8rem; display: flex; align-items: center; gap: 1.4rem; flex-wrap: wrap;
}
.ctrl {
  display: flex; flex-direction: column; gap: 0.4rem;
  font-size: 0.7rem; letter-spacing: 0.14em; text-transform: uppercase; color: var(--ink-soft);
}
.ctrl input[type="range"] { width: 180px; accent-color: var(--deriv); }
.hero-katex { color: var(--integral); font-size: 0.95rem; }
.hero-plot {
  background: var(--paper-2); border: 1px solid var(--line); border-radius: 14px;
  padding: 0.6rem; box-shadow: 0 30px 60px -40px rgba(18, 36, 59, 0.45);
}
.hero-plot svg, .plot svg { width: 100%; height: auto; display: block; border-radius: 10px; }

/* ═══════════ MODULES ═══════════ */
.mod {
  max-width: 1120px; margin: 0 auto;
  padding: clamp(3rem, 8vh, 5rem) var(--pad);
  border-top: 1px solid var(--line);
}
.mod-head { margin-bottom: 2rem; }
.mod-title {
  font-family: var(--font-display); font-weight: 700;
  font-size: clamp(1.8rem, 4.4vw, 3rem); line-height: 1.05; letter-spacing: -0.015em;
}
.mod-title em { font-style: italic; color: var(--deriv); font-weight: 400; }
.mod-sub { margin-top: 0.8rem; font-size: 1.05rem; color: var(--ink-soft); max-width: 62ch; }
.mod-sub .fx { font-family: var(--font-mono); font-size: 0.9em; color: var(--ink); }
.mod-formula { color: var(--integral); margin-top: 0.7rem; font-size: 0.9rem; }
.mod-body {
  display: grid; grid-template-columns: 1.2fr 0.8fr; gap: clamp(1.6rem, 4vw, 2.6rem); align-items: start;
}
.plot {
  background: var(--paper-2); border: 1px solid var(--line); border-radius: 14px;
  padding: 0.6rem; position: relative; touch-action: none;
}
.plot figcaption { margin-top: 0.5rem; font-size: 0.78rem; color: var(--muted); text-align: center; }
.slope-val { color: var(--deriv); font-weight: 600; }
.mod-aside { display: flex; flex-direction: column; gap: 1.1rem; }
.mod-aside p { font-size: 1.02rem; color: var(--ink-soft); }
.mod-aside strong { color: var(--ink); }
.mod-aside em { color: var(--deriv); }
.katex-block {
  background: var(--paper-2); border: 1px solid var(--line); border-left: 3px solid var(--deriv);
  border-radius: 8px; padding: 0.8rem 1rem; overflow-x: auto;
}
.aside-note { font-size: 0.72rem !important; color: var(--muted) !important; letter-spacing: 0.08em; }

/* ═══════════ PRACTICE ═══════════ */
.prob-list { display: flex; flex-direction: column; gap: 1rem; }
.prob {
  background: var(--paper-2); border: 1px solid var(--line); border-radius: 12px;
  padding: 1.2rem 1.4rem;
}
.prob-head { display: flex; align-items: center; gap: 1rem; }
.prob-n {
  color: var(--deriv); font-weight: 600; font-size: 0.9rem;
  border: 1px solid var(--deriv); border-radius: 6px; padding: 0.15rem 0.5rem;
}
.prob-head h3 { font-family: var(--font-display); font-size: 1.15rem; font-weight: 600; flex: 1; }
.hint-btn, .solve-btn {
  font-size: 0.68rem; letter-spacing: 0.1em; text-transform: uppercase;
  border: 1px solid var(--line); border-radius: 999px; background: transparent;
  color: var(--ink-soft); padding: 0.35rem 0.9rem; cursor: pointer;
  transition: color 0.25s ease, border-color 0.25s ease, background 0.25s ease;
}
.hint-btn:hover { color: var(--deriv); border-color: var(--deriv); }
.solve-btn { border-color: var(--integral); color: var(--integral); }
.solve-btn:hover { background: var(--integral); color: var(--paper); }
.prob-q { margin-top: 0.8rem; font-size: 0.92rem; color: var(--ink); }
.prob-hint {
  margin-top: 0.6rem; font-size: 0.82rem; color: var(--deriv);
  background: rgba(232, 116, 46, 0.08); border-left: 2px solid var(--deriv);
  padding: 0.5rem 0.8rem;
}
.prob-proof { margin-top: 0.8rem; border-top: 1px dashed var(--line); padding-top: 0.6rem; }
.proof-line { font-size: 0.88rem; color: var(--ink-soft); padding: 0.18rem 0; opacity: 0.35; }
.proof-line.shown { opacity: 1; color: var(--ink); }
.proof-line.shown::before { content: '› '; color: var(--deriv); }
.proof-ans {
  margin-top: 0.5rem; font-size: 0.95rem; color: var(--integral); font-weight: 600;
  opacity: 0; transition: opacity 0.4s ease;
}
.proof-ans.shown { opacity: 1; }

/* ═══════════ FOOTER ═══════════ */
.site-footer { border-top: 1px solid var(--line); background: var(--paper-2); padding: 1.4rem var(--pad); }
.footer-inner {
  max-width: 1120px; margin: 0 auto;
  display: flex; justify-content: space-between; gap: 1rem; flex-wrap: wrap;
  font-size: 0.74rem; color: var(--muted);
}
.footer-inner a { color: var(--ink-soft); text-decoration: none; }
.footer-inner a:hover { color: var(--deriv); }

/* ═══════════ responsive ═══════════ */
@media (max-width: 900px) {
  .hero { grid-template-columns: 1fr; }
  .mod-body { grid-template-columns: 1fr; }
  .plot { touch-action: auto; }
}
@media (max-width: 560px) {
  body { font-size: 16px; }
  .nav-links { display: none; }
}

/* ═══════════ reduced motion ═══════════ */
@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  * { animation: none !important; transition: none !important; }
}