/* =========================================================
   labcodeku — Shared stylesheet
   Tema: warm cream/ink editorial, accent amber-coral
   ========================================================= */

*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

:root {
  --cream: #faf6ef;
  --cream-2: #f3ede1;
  --cream-3: #ebe4d4;
  --ink: #1a1914;
  --ink-soft: #3d3a33;
  --muted: #7a7469;
  --line: #e5ddcc;
  --line-soft: #eee7d7;
  --accent: #e8632d;
  --accent-soft: #fde8dc;
  --accent-dark: #c84d1c;
  --blue: #2d6fe8;
  --blue-soft: #dce8fd;
  --yellow: #f4c430;
  --yellow-soft: #fff4c9;
  --green: #4a8f3c;
  --green-soft: #dcf0d8;
  --red: #d04848;
  --code-bg: #1a1a24;
  --code-bg-light: #25252f;

  --r-sm: 6px;
  --r-md: 10px;
  --r-lg: 14px;
  --r-xl: 20px;

  --shadow-sm: 0 2px 6px rgba(0,0,0,0.04);
  --shadow-md: 0 8px 24px rgba(0,0,0,0.08);
  --shadow-lg: 0 16px 40px rgba(0,0,0,0.12);
}

html { scroll-behavior: smooth; }

body {
  background: var(--cream);
  color: var(--ink);
  font-family: 'Manrope', -apple-system, sans-serif;
  font-size: 16px;
  line-height: 1.65;
  -webkit-font-smoothing: antialiased;
  overflow-x: hidden;
}

/* =============== TIPOGRAFI =============== */
h1, h2, h3, h4 {
  font-family: 'Fraunces', Georgia, serif;
  font-weight: 600;
  letter-spacing: -0.02em;
  line-height: 1.15;
  color: var(--ink);
}

a { color: var(--accent); text-decoration: none; transition: opacity 0.2s; }
a:hover { opacity: 0.75; }

.mono { font-family: 'JetBrains Mono', monospace; }
em.accent { color: var(--accent); font-style: italic; }

/* =============== LAYOUT UTILS =============== */
.container {
  max-width: 1280px;
  margin: 0 auto;
  padding: 0 2rem;
}

.container-sm {
  max-width: 880px;
  margin: 0 auto;
  padding: 0 2rem;
}

/* =============== NAVBAR =============== */
.navbar {
  position: sticky;
  top: 0;
  background: rgba(250, 246, 239, 0.92);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--line);
  z-index: 100;
  padding: 0.9rem 0;
}

.nav-inner {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 2rem;
}

.logo {
  font-family: 'Fraunces', serif;
  font-weight: 800;
  font-size: 1.4rem;
  letter-spacing: -0.03em;
  display: flex;
  align-items: center;
  gap: 0.5rem;
  color: var(--ink);
}

.logo:hover { opacity: 1; }

.logo-dot {
  width: 9px;
  height: 9px;
  background: var(--accent);
  border-radius: 50%;
  display: inline-block;
  animation: pulse 2s infinite;
}

@keyframes pulse {
  0%, 100% { transform: scale(1); opacity: 1; }
  50% { transform: scale(1.4); opacity: 0.6; }
}

.nav-links {
  display: flex;
  gap: 1.8rem;
  list-style: none;
  align-items: center;
  margin: 0;
}

.nav-links a {
  color: var(--ink-soft);
  font-weight: 500;
  font-size: 0.92rem;
  padding: 0.4rem 0;
  position: relative;
}

.nav-links a:hover { color: var(--accent); opacity: 1; }

.nav-links a.active {
  color: var(--accent);
}

.nav-links a.active::after {
  content: '';
  position: absolute;
  bottom: -0.95rem;
  left: 0;
  right: 0;
  height: 2px;
  background: var(--accent);
}

.menu-toggle {
  display: none;
  background: none;
  border: 1px solid var(--line);
  border-radius: 6px;
  padding: 0.4rem 0.7rem;
  font-size: 1.2rem;
  cursor: pointer;
  color: var(--ink);
}

/* =============== BUTTONS =============== */
.btn {
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  padding: 0.8rem 1.5rem;
  border-radius: var(--r-md);
  font-family: inherit;
  font-weight: 600;
  font-size: 0.93rem;
  border: none;
  cursor: pointer;
  transition: transform 0.15s, box-shadow 0.15s, background 0.2s;
  text-decoration: none;
  line-height: 1;
}

.btn-primary {
  background: var(--ink);
  color: var(--cream);
}

.btn-primary:hover {
  transform: translateY(-2px);
  box-shadow: var(--shadow-md);
  opacity: 1;
}

.btn-secondary {
  background: transparent;
  color: var(--ink);
  border: 1.5px solid var(--ink);
}

.btn-secondary:hover {
  background: var(--ink);
  color: var(--cream);
  opacity: 1;
}

.btn-accent {
  background: var(--accent);
  color: white;
}

.btn-accent:hover {
  background: var(--accent-dark);
  transform: translateY(-2px);
  opacity: 1;
}

.btn-sm {
  padding: 0.5rem 0.9rem;
  font-size: 0.85rem;
}

/* =============== SECTION / LABELS =============== */
.section-label {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.78rem;
  color: var(--accent);
  letter-spacing: 0.15em;
  text-transform: uppercase;
  display: inline-block;
  margin-bottom: 1rem;
}

/* =============== CODE BLOCK =============== */
.code-wrapper {
  border-radius: var(--r-md);
  overflow: hidden;
  background: var(--code-bg);
  box-shadow: var(--shadow-md);
  margin: 1rem 0;
}

.code-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0.65rem 1rem;
  background: var(--code-bg-light);
  border-bottom: 1px solid rgba(255,255,255,0.06);
}

.code-dots { display: flex; gap: 0.4rem; }

.code-dot {
  width: 11px;
  height: 11px;
  border-radius: 50%;
}

.code-dot:nth-child(1) { background: #ff5f57; }
.code-dot:nth-child(2) { background: #febc2e; }
.code-dot:nth-child(3) { background: #28c841; }

.code-filename {
  font-family: 'JetBrains Mono', monospace;
  color: #8b8b95;
  font-size: 0.78rem;
}

.copy-btn {
  background: transparent;
  border: 1px solid rgba(255,255,255,0.1);
  color: #a8a8b5;
  padding: 0.3rem 0.7rem;
  border-radius: 5px;
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.72rem;
  cursor: pointer;
  transition: all 0.2s;
}

.copy-btn:hover {
  background: rgba(255,255,255,0.06);
  color: var(--cream);
}

.copy-btn.copied {
  background: var(--green);
  color: white;
  border-color: var(--green);
}

pre[class*="language-"] {
  margin: 0 !important;
  padding: 1.1rem !important;
  background: var(--code-bg) !important;
  font-size: 0.82rem !important;
  line-height: 1.65 !important;
  overflow-x: auto;
}

code[class*="language-"] {
  font-family: 'JetBrains Mono', monospace !important;
  font-size: 0.82rem !important;
}

/* =============== LIVE DEMO =============== */
.demo-wrapper {
  border: 1px solid var(--line);
  border-radius: var(--r-md);
  overflow: hidden;
  background: white;
  margin-top: 0.75rem;
}

.demo-header {
  padding: 0.55rem 1rem;
  background: var(--cream-2);
  border-bottom: 1px solid var(--line);
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.72rem;
  color: var(--muted);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.demo-body {
  padding: 1.25rem;
  background: white;
  color: #222;
  min-height: 40px;
  font-family: Georgia, serif;
}

.demo-body.interactive { font-family: 'Manrope', sans-serif; }

.demo-body h1, .demo-body h2, .demo-body h3, .demo-body h4 {
  font-family: Arial, sans-serif; color: #222; margin: 0.5em 0; font-weight: bold;
  letter-spacing: normal; line-height: 1.2;
}
.demo-body h1 { font-size: 2em; }
.demo-body h2 { font-size: 1.5em; }
.demo-body h3 { font-size: 1.17em; }
.demo-body h4 { font-size: 1em; }
.demo-body p { margin: 0.5em 0; }
.demo-body ul, .demo-body ol { margin: 0.5em 0 0.5em 1.5em; }
.demo-body a { color: #06c; text-decoration: underline; }
.demo-body table { border-collapse: collapse; margin: 0.5em 0; }
.demo-body table, .demo-body th, .demo-body td { border: 1px solid #aaa; }
.demo-body th, .demo-body td { padding: 6px 12px; }
.demo-body th { background: #eee; }
.demo-body button { cursor: pointer; padding: 6px 14px; border-radius: 4px; border: 1px solid #888; background: white; font-family: inherit; }
.demo-body input, .demo-body textarea, .demo-body select {
  padding: 6px 8px; border: 1px solid #888; border-radius: 4px;
  font-family: inherit; font-size: 1em;
}
.demo-body label { display: block; margin: 0.3em 0; }
.demo-body hr { border: none; border-top: 1px solid #ccc; margin: 0.75em 0; }
.demo-body blockquote { border-left: 3px solid #888; padding-left: 1em; margin: 0.5em 0; color: #555; font-style: italic; }
.demo-body code { background: #f5f5f5; padding: 2px 5px; border-radius: 3px; font-family: monospace; }

/* =============== NOTES (Tip / Warn / Info) =============== */
.tip, .warn, .info {
  padding: 1rem 1.25rem;
  border-radius: 0 var(--r-md) var(--r-md) 0;
  margin: 1rem 0;
  font-size: 0.94rem;
  line-height: 1.55;
}

.tip { background: var(--yellow-soft); border-left: 3px solid var(--yellow); }
.tip strong { color: #8a6d00; display: block; margin-bottom: 0.3rem; font-size: 0.78rem; letter-spacing: 0.08em; text-transform: uppercase; font-weight: 700; }

.warn { background: var(--accent-soft); border-left: 3px solid var(--accent); }
.warn strong { color: var(--accent-dark); display: block; margin-bottom: 0.3rem; font-size: 0.78rem; letter-spacing: 0.08em; text-transform: uppercase; font-weight: 700; }

.info { background: var(--blue-soft); border-left: 3px solid var(--blue); }
.info strong { color: var(--blue); display: block; margin-bottom: 0.3rem; font-size: 0.78rem; letter-spacing: 0.08em; text-transform: uppercase; font-weight: 700; }

.success { background: var(--green-soft); border-left: 3px solid var(--green); padding: 1rem 1.25rem; border-radius: 0 var(--r-md) var(--r-md) 0; margin: 1rem 0; }
.success strong { color: var(--green); display: block; margin-bottom: 0.3rem; font-size: 0.78rem; letter-spacing: 0.08em; text-transform: uppercase; font-weight: 700; }

/* =============== INLINE CODE (dalam paragraf) =============== */
p code, li code, td code, h3 code, h4 code, .tip code, .warn code, .info code, .success code {
  background: var(--accent-soft);
  color: var(--accent-dark);
  padding: 0.12rem 0.4rem;
  border-radius: 4px;
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.86em;
  font-weight: 500;
}

/* =============== LESSON LAYOUT (dengan sidebar) =============== */
.lesson-layout {
  display: grid;
  grid-template-columns: 260px 1fr;
  gap: 3rem;
  padding: 2.5rem 0 4rem;
  max-width: 1280px;
  margin: 0 auto;
  padding-left: 2rem;
  padding-right: 2rem;
}

.sidebar {
  position: sticky;
  top: 90px;
  height: calc(100vh - 110px);
  overflow-y: auto;
  padding-right: 0.5rem;
  font-size: 0.9rem;
}

.sidebar::-webkit-scrollbar { width: 6px; }
.sidebar::-webkit-scrollbar-track { background: transparent; }
.sidebar::-webkit-scrollbar-thumb { background: var(--line); border-radius: 3px; }

.sidebar-title {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.72rem;
  color: var(--muted);
  letter-spacing: 0.15em;
  text-transform: uppercase;
  margin-bottom: 1rem;
  padding-bottom: 0.6rem;
  border-bottom: 1px solid var(--line);
}

.sidebar ol {
  list-style: none;
  counter-reset: lesson;
}

.sidebar li {
  counter-increment: lesson;
  margin-bottom: 0.1rem;
}

.sidebar a {
  display: flex;
  align-items: flex-start;
  gap: 0.6rem;
  padding: 0.55rem 0.7rem;
  color: var(--ink-soft);
  font-weight: 500;
  border-radius: 6px;
  border-left: 2px solid transparent;
  line-height: 1.4;
  transition: all 0.15s;
  font-size: 0.88rem;
}

.sidebar a::before {
  content: counter(lesson, decimal-leading-zero);
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.72rem;
  color: var(--muted);
  flex-shrink: 0;
  padding-top: 1px;
}

.sidebar a:hover {
  background: var(--cream-2);
  color: var(--ink);
  opacity: 1;
}

.sidebar a.active {
  background: var(--accent-soft);
  color: var(--accent-dark);
  border-left-color: var(--accent);
}

.sidebar a.completed::after {
  content: '✓';
  margin-left: auto;
  color: var(--green);
  font-weight: 700;
}

/* =============== LESSON CONTENT =============== */
.lesson-main {
  min-width: 0;
  max-width: 860px;
}

.page-header {
  margin-bottom: 3rem;
  padding-bottom: 2rem;
  border-bottom: 1px solid var(--line);
}

.page-header h1 {
  font-size: clamp(2.2rem, 5vw, 3.5rem);
  margin-bottom: 0.75rem;
  letter-spacing: -0.03em;
}

.page-header .subtitle {
  font-size: 1.1rem;
  color: var(--ink-soft);
  max-width: 640px;
}

.lesson {
  padding: 2.5rem 0;
  border-bottom: 1px solid var(--line-soft);
  scroll-margin-top: 90px;
}

.lesson:last-child { border-bottom: none; }

.lesson-number {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.78rem;
  color: var(--accent);
  letter-spacing: 0.1em;
  display: block;
  margin-bottom: 0.5rem;
}

.lesson h2 {
  font-size: clamp(1.6rem, 3vw, 2.2rem);
  margin-bottom: 1rem;
  letter-spacing: -0.02em;
}

.lesson p {
  color: var(--ink-soft);
  margin-bottom: 1rem;
  max-width: 68ch;
}

.lesson ul, .lesson ol {
  margin: 1rem 0 1rem 1.5rem;
  color: var(--ink-soft);
}

.lesson li { margin-bottom: 0.4rem; }

.lesson h3, .lesson h4 {
  margin-top: 1.75rem;
  margin-bottom: 0.75rem;
  font-size: 1.25rem;
}

.lesson h4 { font-size: 1.1rem; }

.lesson strong { color: var(--ink); font-weight: 700; }

.lesson table {
  border-collapse: collapse;
  margin: 1rem 0;
  width: 100%;
  font-size: 0.92rem;
}

.lesson table th, .lesson table td {
  border: 1px solid var(--line);
  padding: 0.55rem 0.8rem;
  text-align: left;
  vertical-align: top;
}

.lesson table th {
  background: var(--cream-2);
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.82rem;
  letter-spacing: 0.05em;
  text-transform: uppercase;
  color: var(--ink-soft);
  font-weight: 600;
}

/* Mark complete button */
.mark-complete {
  margin-top: 1.25rem;
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  padding: 0.55rem 1rem;
  border: 1.5px solid var(--line);
  background: transparent;
  border-radius: var(--r-md);
  color: var(--ink-soft);
  font-family: inherit;
  font-size: 0.85rem;
  font-weight: 600;
  cursor: pointer;
  transition: all 0.2s;
}

.mark-complete:hover {
  border-color: var(--green);
  color: var(--green);
}

.mark-complete.done {
  background: var(--green-soft);
  border-color: var(--green);
  color: var(--green);
}

/* Two column layout inside lesson */
.two-col {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 1.5rem;
  align-items: start;
  margin: 1rem 0;
}

/* =============== EXERCISE =============== */
.exercise {
  background: var(--cream-2);
  border: 1px solid var(--line);
  border-radius: var(--r-lg);
  padding: 1.75rem 2rem;
  margin: 2rem 0;
}

.exercise-label {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.72rem;
  letter-spacing: 0.15em;
  text-transform: uppercase;
  color: var(--accent);
  margin-bottom: 0.5rem;
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

.exercise-label::before {
  content: '▸';
  font-size: 0.85rem;
}

.exercise h3 {
  margin: 0 0 0.75rem;
  font-size: 1.3rem;
}

.exercise details {
  margin-top: 1rem;
  border-top: 1px dashed var(--line);
  padding-top: 1rem;
}

.exercise summary {
  cursor: pointer;
  font-weight: 600;
  color: var(--accent);
  font-size: 0.9rem;
  padding: 0.4rem 0;
  user-select: none;
  list-style: none;
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

.exercise summary::-webkit-details-marker { display: none; }
.exercise summary::before { content: '▶'; font-size: 0.7rem; transition: transform 0.2s; }
.exercise details[open] summary::before { transform: rotate(90deg); }
.exercise summary:hover { opacity: 0.75; }

/* =============== QUIZ =============== */
.quiz {
  background: var(--yellow-soft);
  border: 1px solid #f0e29f;
  border-radius: var(--r-lg);
  padding: 1.5rem 1.75rem;
  margin: 2rem 0;
}

.quiz-label {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.72rem;
  letter-spacing: 0.15em;
  text-transform: uppercase;
  color: #8a6d00;
  margin-bottom: 0.5rem;
}

.quiz h3 {
  font-family: 'Fraunces', serif;
  font-size: 1.15rem;
  margin-bottom: 1rem;
  color: var(--ink);
}

.quiz-options {
  display: flex;
  flex-direction: column;
  gap: 0.5rem;
}

.quiz-option {
  padding: 0.75rem 1rem;
  background: white;
  border: 1.5px solid var(--line);
  border-radius: var(--r-md);
  cursor: pointer;
  font-family: inherit;
  font-size: 0.93rem;
  text-align: left;
  transition: all 0.15s;
  color: var(--ink);
  width: 100%;
}

.quiz-option:hover {
  border-color: var(--ink-soft);
  transform: translateX(3px);
}

.quiz-option.correct {
  background: var(--green-soft);
  border-color: var(--green);
  color: var(--green);
  font-weight: 600;
}

.quiz-option.wrong {
  background: #fde0e0;
  border-color: var(--red);
  color: var(--red);
}

.quiz-feedback {
  margin-top: 1rem;
  padding: 0.75rem 1rem;
  border-radius: var(--r-md);
  font-size: 0.92rem;
  display: none;
}

.quiz-feedback.show { display: block; }
.quiz-feedback.correct { background: var(--green-soft); color: var(--green); }
.quiz-feedback.wrong { background: #fde0e0; color: var(--red); }

/* =============== TABS =============== */
.tabs {
  display: flex;
  border-bottom: 1px solid var(--line);
  margin-bottom: 1rem;
  overflow-x: auto;
}

.tab-btn {
  background: transparent;
  border: none;
  padding: 0.7rem 1.15rem;
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.82rem;
  color: var(--muted);
  cursor: pointer;
  border-bottom: 2px solid transparent;
  transition: all 0.2s;
  margin-bottom: -1px;
  white-space: nowrap;
}

.tab-btn.active {
  color: var(--ink);
  border-bottom-color: var(--accent);
}

.tab-btn:hover:not(.active) { color: var(--ink-soft); }

.tab-content { display: none; }
.tab-content.active { display: block; }

/* =============== FOOTER NAV (prev / next) =============== */
.lesson-foot-nav {
  margin-top: 3rem;
  padding-top: 2rem;
  border-top: 2px solid var(--line);
  display: flex;
  justify-content: space-between;
  gap: 1rem;
  flex-wrap: wrap;
}

.foot-nav-link {
  flex: 1;
  min-width: 240px;
  padding: 1.25rem 1.5rem;
  background: var(--cream-2);
  border: 1px solid var(--line);
  border-radius: var(--r-lg);
  transition: all 0.2s;
  display: block;
}

.foot-nav-link:hover {
  transform: translateY(-3px);
  border-color: var(--ink);
  box-shadow: var(--shadow-md);
  opacity: 1;
}

.foot-nav-link.next { text-align: right; }

.foot-nav-link .fn-label {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.72rem;
  color: var(--muted);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  display: block;
  margin-bottom: 0.3rem;
}

.foot-nav-link .fn-title {
  font-family: 'Fraunces', serif;
  font-weight: 600;
  color: var(--ink);
  font-size: 1.1rem;
}

/* =============== FOOTER =============== */
.footer {
  background: var(--ink);
  color: #c0bdb5;
  padding: 3.5rem 0 2rem;
}

.footer-grid {
  display: grid;
  grid-template-columns: 2fr 1fr 1fr 1fr;
  gap: 2.5rem;
  margin-bottom: 3rem;
}

.footer-brand .logo { color: var(--cream); margin-bottom: 1rem; }
.footer-brand p { color: #8a867e; max-width: 320px; font-size: 0.94rem; }

.footer h4 {
  color: var(--cream);
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.78rem;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  margin-bottom: 1rem;
}

.footer ul { list-style: none; }
.footer li { margin-bottom: 0.5rem; }
.footer a { color: #c0bdb5; font-size: 0.9rem; }
.footer a:hover { color: var(--accent); opacity: 1; }

.footer-bottom {
  border-top: 1px solid #2d2b26;
  padding-top: 1.5rem;
  text-align: center;
  color: #6b6860;
  font-size: 0.82rem;
  font-family: 'JetBrains Mono', monospace;
}

.footer-bottom .heart { color: var(--accent); }

/* =============== SCROLL TO TOP =============== */
.scroll-top {
  position: fixed;
  bottom: 2rem;
  right: 2rem;
  width: 44px;
  height: 44px;
  border-radius: 50%;
  background: var(--ink);
  color: var(--cream);
  border: none;
  cursor: pointer;
  display: none;
  align-items: center;
  justify-content: center;
  font-size: 1.1rem;
  box-shadow: var(--shadow-lg);
  z-index: 50;
  transition: transform 0.2s;
}

.scroll-top:hover { transform: translateY(-3px); }
.scroll-top.visible { display: flex; }

/* =============== MOBILE SIDEBAR TOGGLE =============== */
.sidebar-toggle {
  display: none;
  width: 100%;
  padding: 0.75rem 1rem;
  background: var(--ink);
  color: var(--cream);
  border: none;
  border-radius: var(--r-md);
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.82rem;
  font-weight: 600;
  cursor: pointer;
  margin-bottom: 1rem;
  text-align: left;
  display: none;
  justify-content: space-between;
  align-items: center;
}

/* =============== RESPONSIVE =============== */
@media (max-width: 960px) {
  .lesson-layout { grid-template-columns: 1fr; gap: 0; }
  .sidebar {
    position: static;
    height: auto;
    max-height: 0;
    overflow: hidden;
    transition: max-height 0.3s;
    padding: 0;
  }
  .sidebar.open {
    max-height: 2000px;
    padding: 1rem 0 1.5rem;
  }
  .sidebar-toggle {
    display: flex;
    margin-top: 1rem;
  }
  .footer-grid { grid-template-columns: 1fr 1fr; gap: 2rem; }
  .two-col { grid-template-columns: 1fr; }
}

@media (max-width: 640px) {
  .container, .container-sm { padding: 0 1.25rem; }
  .lesson-layout { padding-left: 1.25rem; padding-right: 1.25rem; }
  .menu-toggle { display: block; }
  .nav-links {
    display: none;
    position: absolute;
    top: 100%;
    left: 0;
    right: 0;
    background: var(--cream);
    flex-direction: column;
    align-items: stretch;
    padding: 1rem 1.5rem;
    border-bottom: 1px solid var(--line);
    gap: 0.3rem;
  }
  .nav-links a {
    padding: 0.65rem 0;
    border-bottom: 1px solid var(--line-soft);
  }
  .nav-links a.active::after { display: none; }
  .nav-links.open { display: flex; }
  .footer-grid { grid-template-columns: 1fr; }
  .lesson { padding: 2rem 0; }
  .exercise { padding: 1.25rem; }
}

/* =============== UTILITY =============== */
.text-center { text-align: center; }
.mt-0 { margin-top: 0; }
.mt-1 { margin-top: 0.5rem; }
.mt-2 { margin-top: 1rem; }
.mt-3 { margin-top: 1.5rem; }
.mt-4 { margin-top: 2rem; }