:root {
  /* === Monochrome black & white professional palette === */
  --bg: #ffffff;
  --bg-soft: #f6f6f7;
  --bg-elev: #ffffff;
  --bg-subtle: #f0f0f1;
  --bg-hover: #ececee;

  --border: #e3e3e5;
  --border-strong: #c7c7cc;

  --text-primary: #111114;
  --text-secondary: #3f3f46;
  --text-tertiary: #6b6b72;
  --text-muted: #9b9ba2;

  /* --- Primary: near-black (monochrome) --- */
  --accent: #111114;
  --accent-soft: #efefef;
  --accent-strong: #000000;
  --accent-text: #111114;

  /* Semantic colors also neutralized */
  --success: #111114;
  --success-soft: #ececee;
  --warn: #52525b;
  --warn-soft: #f0f0f1;
  --danger: #27272a;
  --danger-soft: #ececee;
  --info: #52525b;

  /* --- Category colors unified to monochrome --- */
  --acoustics: #111114;
  --optics: #111114;
  --thermal: #111114;
  --mechanics: #111114;
  --electricity: #111114;
  --electromagnetism: #111114;

  --shadow-sm: 0 1px 2px 0 rgba(0,0,0,0.04), 0 1px 3px 0 rgba(0,0,0,0.03);
  --shadow-md: 0 4px 12px -2px rgba(0,0,0,0.07), 0 2px 6px -2px rgba(0,0,0,0.04);
  --shadow-lg: 0 20px 40px -12px rgba(0,0,0,0.14), 0 8px 16px -4px rgba(0,0,0,0.06);
  --shadow-xl: 0 25px 50px -12px rgba(0,0,0,0.16), 0 10px 20px -5px rgba(0,0,0,0.08);

  --radius-sm: 6px;
  --radius-md: 10px;
  --radius-lg: 16px;
  --radius-xl: 20px;
}

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

html, body, #root {
  width: 100%;
  height: 100%;
  overflow: hidden;
  font-family: 'Inter', 'Noto Sans SC', -apple-system, BlinkMacSystemFont, 'PingFang SC', sans-serif;
  color: var(--text-primary);
  background: var(--bg);
  -webkit-font-smoothing: antialiased;
  font-feature-settings: 'cv11', 'ss01';
}

button {
  font-family: inherit;
  cursor: pointer;
  border: none;
  background: none;
  color: inherit;
}

input, select { font-family: inherit; }

/* ========== App Shell ========== */
.app {
  width: 100vw;
  height: 100vh;
  display: flex;
  flex-direction: column;
  background: var(--bg);
}

.app-header {
  height: 64px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 40px;
  background: #ffffff;
  border-bottom: 1px solid var(--border);
  flex-shrink: 0;
  backdrop-filter: saturate(180%) blur(8px);
  position: relative;
  z-index: 10;
}

.app-title {
  display: flex;
  align-items: center;
  gap: 10px;
  font-size: 16px;
  font-weight: 700;
  letter-spacing: -0.2px;
  color: var(--accent-text);
}

.app-title-icon {
  width: 34px;
  height: 34px;
  border-radius: 9px;
  background: linear-gradient(135deg, var(--accent) 0%, var(--accent-strong) 100%);
  display: flex;
  align-items: center;
  justify-content: center;
  color: #fff;
  box-shadow: 0 2px 8px rgba(0,0,0,0.25);
}

.app-title-icon svg {
  width: 20px;
  height: 20px;
  stroke: #fff;
  stroke-width: 1.6;
  fill: none;
  stroke-linecap: round;
  stroke-linejoin: round;
}

/* Top nav center links */
.nav-links {
  display: flex;
  align-items: center;
  gap: 4px;
}

.nav-link {
  padding: 6px 14px;
  font-size: 13.5px;
  color: var(--text-secondary);
  border-radius: 8px;
  transition: all .15s;
  font-weight: 500;
  cursor: pointer;
}

.nav-link:hover {
  color: var(--accent-text);
  background: var(--accent-soft);
}

.breadcrumb {
  display: flex;
  align-items: center;
  gap: 8px;
  font-size: 13px;
  font-weight: 500;
  margin-left: 4px;
}

.bc-link {
  color: var(--text-tertiary);
  cursor: pointer;
  transition: color .15s;
  font-weight: 500;
}
.bc-link:hover { color: var(--accent); }

.bc-sep {
  color: var(--text-muted);
  font-size: 11px;
}

.bc-current {
  color: var(--text-primary);
  font-weight: 600;
}

.app-header-actions {
  display: flex;
  align-items: center;
  gap: 12px;
}

/* Primary CTA button in header */
.btn-cta {
  padding: 9px 20px;
  background: var(--accent);
  color: #fff;
  border-radius: 10px;
  font-size: 13.5px;
  font-weight: 600;
  transition: all .2s;
  display: inline-flex;
  align-items: center;
  gap: 6px;
  border: 1px solid var(--accent);
  box-shadow: 0 1px 3px rgba(0,0,0,0.2);
}

.btn-cta:hover {
  background: var(--accent-strong);
  border-color: var(--accent-strong);
  transform: translateY(-1px);
  box-shadow: 0 4px 12px rgba(0,0,0,0.3);
}

.btn-ghost {
  padding: 9px 18px;
  background: #fff;
  color: var(--text-secondary);
  border-radius: 10px;
  font-size: 13.5px;
  font-weight: 500;
  transition: all .15s;
  display: inline-flex;
  align-items: center;
  gap: 6px;
  border: 1px solid var(--border);
}

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

.back-btn {
  display: flex;
  align-items: center;
  gap: 6px;
  padding: 7px 16px;
  background: var(--bg-soft);
  border: 1px solid var(--border);
  border-radius: 8px;
  font-size: 13px;
  color: var(--text-secondary);
  transition: all .15s;
  font-weight: 500;
}
.back-btn:hover {
  background: var(--bg-elev);
  border-color: var(--border-strong);
  color: var(--text-primary);
}

/* ========== Portal / Home Page ========== */
.portal {
  flex: 1;
  overflow-y: auto;
  position: relative;
  background: #fff;
}

.portal-home {
  display: block;
}

/* --- Hero Section --- */
.hero-section {
  position: relative;
  z-index: 2;
  flex-shrink: 0;
  padding: 80px 40px 90px;
  overflow: hidden;
  background: linear-gradient(180deg, #fafafa 0%, #ffffff 100%);
}

.hero-grid {
  max-width: 1200px;
  margin: 0 auto;
  display: grid;
  grid-template-columns: 1.05fr 1fr;
  gap: 60px;
  align-items: center;
  position: relative;
  z-index: 3;
}

.hero-left {
  display: flex;
  flex-direction: column;
  gap: 0;
}

.hero-badge {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  padding: 6px 16px;
  background: var(--accent-soft);
  border-radius: 100px;
  font-size: 13px;
  color: var(--accent-text);
  font-weight: 500;
  margin-bottom: 24px;
  width: fit-content;
  letter-spacing: 0.2px;
}

.hero-badge svg {
  width: 14px;
  height: 14px;
  stroke: currentColor;
  stroke-width: 2;
  fill: none;
  stroke-linecap: round;
  stroke-linejoin: round;
  flex-shrink: 0;
}

.hero-title {
  font-size: 52px !important;
  font-weight: 700 !important;
  line-height: 1.15 !important;
  letter-spacing: -0.8px !important;
  color: var(--text-primary) !important;
  margin-bottom: 20px !important;
  background: none !important;
  -webkit-text-fill-color: var(--text-primary) !important;
  text-align: left !important;
  font-family: 'Inter', 'Noto Sans SC', sans-serif !important;
}

.hero-title .highlight {
  color: var(--accent);
  -webkit-text-fill-color: var(--accent);
  position: relative;
  text-decoration: underline;
  text-decoration-color: var(--text-primary);
  text-decoration-thickness: 3px;
  text-underline-offset: 9px;
}

.hero-desc {
  font-size: 17px;
  line-height: 1.7;
  color: var(--text-secondary);
  margin-bottom: 32px;
  max-width: 520px;
  font-weight: 400;
}

.hero-buttons {
  display: flex;
  gap: 12px;
  margin-bottom: 28px;
  flex-wrap: wrap;
}

.hero-btn-primary {
  padding: 13px 26px;
  background: var(--accent);
  color: #fff;
  border-radius: 11px;
  font-size: 14.5px;
  font-weight: 600;
  transition: all .2s;
  display: inline-flex;
  align-items: center;
  gap: 8px;
  border: 1px solid var(--accent);
  box-shadow: 0 2px 8px rgba(0,0,0,0.25);
  cursor: pointer;
}

.hero-btn-primary:hover {
  background: var(--accent-strong);
  border-color: var(--accent-strong);
  transform: translateY(-2px);
  box-shadow: 0 8px 20px rgba(0,0,0,0.3);
}

.hero-btn-primary svg {
  width: 16px;
  height: 16px;
  stroke: currentColor;
  stroke-width: 2.2;
  fill: none;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.hero-btn-secondary {
  padding: 13px 24px;
  background: #fff;
  color: var(--text-primary);
  border-radius: 11px;
  font-size: 14.5px;
  font-weight: 500;
  transition: all .2s;
  display: inline-flex;
  align-items: center;
  gap: 8px;
  border: 1px solid var(--border-strong);
  cursor: pointer;
}

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

.hero-btn-secondary svg {
  width: 15px;
  height: 15px;
  stroke: currentColor;
  stroke-width: 2;
  fill: none;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.hero-features {
  display: flex;
  flex-wrap: wrap;
  gap: 16px 24px;
}

.hero-feature {
  display: flex;
  align-items: center;
  gap: 8px;
  font-size: 13.5px;
  color: var(--text-secondary);
  font-weight: 500;
}

.hero-feature svg {
  width: 16px;
  height: 16px;
  stroke: var(--success);
  stroke-width: 2.5;
  fill: none;
  stroke-linecap: round;
  stroke-linejoin: round;
  flex-shrink: 0;
}

/* --- Hero Preview Card (browser frame) --- */
.hero-right {
  display: flex;
  justify-content: center;
  align-items: center;
  position: relative;
}

.hero-preview-stack {
  position: relative;
  width: 100%;
  max-width: 560px;
}

.hero-preview-card {
  background: #fff;
  border-radius: 16px;
  box-shadow: var(--shadow-xl);
  overflow: hidden;
  border: 1px solid var(--border);
  position: relative;
  z-index: 2;
}

.hero-preview-card.back {
  position: absolute;
  top: 20px;
  right: -10px;
  left: 30px;
  z-index: 1;
  opacity: 0.5;
  transform: rotate(2deg) scale(0.95);
}

.preview-header {
  height: 40px;
  display: flex;
  align-items: center;
  padding: 0 16px;
  background: #f6f6f7;
  border-bottom: 1px solid var(--border);
  gap: 8px;
}

.preview-dots {
  display: flex;
  gap: 6px;
}

.preview-dot {
  width: 10px;
  height: 10px;
  border-radius: 50%;
}

.preview-dot.red { background: #d4d4d8; }
.preview-dot.yellow { background: #a1a1aa; }
.preview-dot.green { background: #71717a; }

.preview-title {
  margin-left: 12px;
  font-size: 11px;
  color: var(--text-tertiary);
  font-family: 'JetBrains Mono', monospace;
  font-weight: 500;
}

.preview-body {
  padding: 0;
  background: #fff;
}

.preview-body svg {
  width: 100%;
  height: auto;
  display: block;
}

/* --- Categories Section --- */
.categories-section {
  padding: 80px 40px 100px;
  background: #fff;
  position: relative;
  z-index: 2;
  flex-shrink: 0;
}

.categories-header {
  position: relative;
  z-index: 1;
  text-align: center;
  max-width: 640px;
  margin: 0 auto 56px;
}

.section-eyebrow {
  font-size: 13px;
  font-weight: 600;
  color: var(--accent);
  letter-spacing: 1px;
  text-transform: uppercase;
  margin-bottom: 14px;
  font-family: 'Inter', sans-serif;
}

.section-title {
  font-size: 36px;
  font-weight: 700;
  color: var(--text-primary);
  letter-spacing: -0.5px;
  line-height: 1.25;
  margin-bottom: 14px;
}

.section-desc {
  font-size: 16px;
  color: var(--text-secondary);
  line-height: 1.6;
}

.categories-grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 20px;
  max-width: 1200px;
  margin: 0 auto;
  position: relative;
  z-index: 2;
  width: 100%;
}

.category-card {
  background: #fff;
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  padding: 32px 28px 28px;
  transition: all .25s cubic-bezier(0.4, 0, 0.2, 1);
  cursor: pointer;
  box-shadow: var(--shadow-sm);
  position: relative;
  overflow: hidden;
}

.category-card::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  height: 3px;
  background: var(--cat-color, var(--accent));
  opacity: 0;
  transition: opacity .25s;
}

.category-card:hover {
  transform: translateY(-4px);
  border-color: var(--cat-color, var(--accent));
  box-shadow: var(--shadow-lg);
}

.category-card:hover::before {
  opacity: 1;
}

.category-icon-wrap {
  width: 52px;
  height: 52px;
  border-radius: 13px;
  background: var(--cat-color, var(--accent));
  display: flex;
  align-items: center;
  justify-content: center;
  margin-bottom: 22px;
  color: #fff;
  transition: transform .3s;
}

.category-card:hover .category-icon-wrap {
  transform: scale(1.06) rotate(-3deg);
}

.category-icon-wrap svg {
  width: 26px;
  height: 26px;
  stroke: #fff;
  stroke-width: 1.8;
  fill: none;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.category-name {
  font-size: 20px;
  font-weight: 700;
  color: var(--text-primary);
  letter-spacing: 0.2px;
  margin-bottom: 6px;
}

.category-english {
  font-size: 11px;
  color: var(--text-tertiary);
  letter-spacing: 1.5px;
  text-transform: uppercase;
  margin-bottom: 12px;
  font-weight: 500;
}

.category-count {
  font-size: 13px;
  color: var(--cat-color, var(--accent));
  font-weight: 600;
  display: flex;
  align-items: center;
  gap: 6px;
  transition: gap .2s;
}

.category-count svg {
  width: 14px;
  height: 14px;
  stroke: currentColor;
  stroke-width: 2.2;
  fill: none;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.category-card:hover .category-count {
  gap: 10px;
}

/* ========== Category Page ========== */
.category-page {
  padding: 0;
  background: var(--bg-soft);
}

.cat-hero {
  padding: 56px 40px 40px;
  max-width: 1200px;
  margin: 0 auto;
  width: 100%;
  display: flex;
  align-items: center;
  gap: 24px;
  position: relative;
  z-index: 2;
}

.cat-hero-icon {
  width: 72px;
  height: 72px;
  border-radius: 18px;
  background: linear-gradient(135deg, var(--cat-color, var(--accent)) 0%, var(--cat-color, var(--accent-strong)) 100%);
  filter: brightness(1.05);
  display: flex;
  align-items: center;
  justify-content: center;
  color: #fff;
  flex-shrink: 0;
  box-shadow: 0 8px 24px rgba(0,0,0,0.1);
}

.cat-hero-icon svg {
  width: 36px;
  height: 36px;
  stroke: #fff;
  stroke-width: 1.6;
  fill: none;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.cat-hero-text {
  flex: 1;
}

.cat-hero-name {
  font-size: 36px;
  font-weight: 700;
  color: var(--text-primary);
  letter-spacing: -0.3px;
  margin-bottom: 4px;
}

.cat-hero-english {
  font-size: 12px;
  color: var(--text-tertiary);
  letter-spacing: 1.5px;
  text-transform: uppercase;
  margin-bottom: 8px;
  font-weight: 500;
}

.cat-hero-desc {
  font-size: 14px;
  color: var(--text-secondary);
}

.exp-cards-grid {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: 14px;
  max-width: 1200px;
  margin: 24px auto 60px;
  padding: 0 40px;
  position: relative;
  z-index: 2;
  width: 100%;
}

.exp-card {
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 22px 26px;
  display: flex;
  align-items: center;
  gap: 18px;
  transition: all .2s ease;
  cursor: pointer;
  box-shadow: var(--shadow-sm);
}

.exp-card:hover {
  transform: translateX(4px);
  border-color: var(--cat-color, var(--accent));
  box-shadow: var(--shadow-md);
  background: var(--bg-elev);
}

.exp-card-num {
  width: 38px;
  height: 38px;
  border-radius: 9px;
  background: var(--bg-soft);
  display: flex;
  align-items: center;
  justify-content: center;
  font-family: 'JetBrains Mono', monospace;
  font-size: 13px;
  font-weight: 600;
  color: var(--text-tertiary);
  flex-shrink: 0;
  transition: all .2s;
}

.exp-card:hover .exp-card-num {
  background: var(--cat-color, var(--accent));
  color: #fff;
}

.exp-card-name {
  flex: 1;
  font-size: 15px;
  font-weight: 500;
  color: var(--text-primary);
}

.exp-card-arrow {
  color: var(--text-muted);
  font-size: 18px;
  opacity: 0.5;
  transition: all .2s;
  flex-shrink: 0;
}

.exp-card-arrow svg {
  width: 18px;
  height: 18px;
  stroke: currentColor;
  stroke-width: 2;
  fill: none;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.exp-card:hover .exp-card-arrow {
  opacity: 1;
  color: var(--cat-color, var(--accent));
}

.category-card {
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  overflow: hidden;
  transition: all .25s ease;
  cursor: pointer;
  box-shadow: var(--shadow-sm);
}

.category-card:hover {
  transform: translateY(-3px);
  border-color: var(--cat-color, var(--accent));
  box-shadow: var(--shadow-lg);
}

.category-card-header {
  height: 76px;
  border-bottom: 1px solid var(--border);
  display: flex;
  align-items: center;
  gap: 14px;
  padding: 0 20px;
  position: relative;
}

.category-card-header::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  height: 3px;
  background: var(--cat-color, var(--accent));
  opacity: 0.7;
}

.category-icon {
  width: 42px;
  height: 42px;
  border-radius: 10px;
  background: var(--cat-color, var(--accent));
  opacity: 0.9;
  display: flex;
  align-items: center;
  justify-content: center;
  color: #fff;
  flex-shrink: 0;
}

.category-title {
  font-size: 18px;
  font-weight: 600;
  color: var(--text-primary);
  letter-spacing: 1px;
}

.category-subtitle {
  font-size: 11.5px;
  color: var(--text-tertiary);
  margin-top: 3px;
  letter-spacing: 0.5px;
  text-transform: uppercase;
}

.category-experiments {
  padding: 8px;
  display: flex;
  flex-direction: column;
  gap: 2px;
}

.exp-item {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 9px 12px;
  border-radius: 6px;
  font-size: 13px;
  color: var(--text-secondary);
  transition: all .15s;
  cursor: pointer;
}

.exp-item:hover {
  background: var(--bg-soft);
  color: var(--text-primary);
}

.exp-item-dot {
  width: 5px;
  height: 5px;
  border-radius: 50%;
  background: var(--cat-color, var(--accent));
  flex-shrink: 0;
  opacity: 0.7;
}

.exp-count {
  margin-left: auto;
  font-size: 11px;
  color: var(--text-muted);
  font-family: 'JetBrains Mono', monospace;
}

/* ========== Floating formula background ========== */
.formula-bg {
  position: absolute;
  inset: 0;
  overflow: hidden;
  pointer-events: none;
  z-index: 0;
}

.formula-item {
  position: absolute;
  font-family: 'JetBrains Mono', 'Cambria Math', serif;
  font-size: 18px;
  color: var(--accent);
  opacity: 0.045;
  font-style: italic;
  letter-spacing: 0.5px;
  animation: floatFormula 28s ease-in-out infinite;
  user-select: none;
  font-weight: 400;
}

@keyframes floatFormula {
  0%, 100% {
    transform: translateY(0) rotate(0deg);
    opacity: 0.03;
  }
  50% {
    transform: translateY(-15px) rotate(0.5deg);
    opacity: 0.06;
  }
}

/* Soft grid pattern for hero background */
.hero-bg-pattern {
  position: absolute;
  inset: 0;
  pointer-events: none;
  z-index: 0;
  background-image:
    radial-gradient(circle at 80% 20%, rgba(0, 0, 0, 0.08) 0%, transparent 50%),
    radial-gradient(circle at 20% 80%, rgba(0, 0, 0, 0.05) 0%, transparent 50%);
}

.hero-grid-dots {
  position: absolute;
  inset: 0;
  pointer-events: none;
  z-index: 0;
  background-image: radial-gradient(circle, #cbd5e1 1px, transparent 1px);
  background-size: 28px 28px;
  opacity: 0.3;
  mask-image: radial-gradient(ellipse at center, black 30%, transparent 70%);
  -webkit-mask-image: radial-gradient(ellipse at center, black 30%, transparent 70%);
}

/* ========== Experiment Page Layout ========== */
.experiment {
  flex: 1;
  display: grid;
  grid-template-columns: 1fr 380px;
  overflow: hidden;
}

.exp-main {
  display: flex;
  flex-direction: column;
  overflow: hidden;
  min-width: 0;
  background: var(--bg-soft);
}

.exp-header {
  height: 52px;
  display: flex;
  align-items: center;
  padding: 0 24px;
  background: var(--bg-elev);
  border-bottom: 1px solid var(--border);
  gap: 12px;
  flex-shrink: 0;
}

.exp-category-tag {
  padding: 4px 12px;
  border-radius: 6px;
  background: var(--cat-color, var(--accent));
  color: #fff;
  font-size: 12px;
  font-weight: 500;
  opacity: 0.9;
  letter-spacing: 0.3px;
}

.exp-name {
  font-size: 15px;
  font-weight: 600;
  color: var(--text-primary);
  letter-spacing: 0.2px;
}

.sim-area {
  flex: 1;
  position: relative;
  background: var(--bg-soft);
  overflow: hidden;
  min-height: 0;
}

.sim-canvas-wrap {
  width: 100%;
  height: 100%;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 24px;
}

.sim-canvas {
  background: #fff;
  border-radius: 12px;
  box-shadow: var(--shadow-md);
  border: 1px solid var(--border);
}

.sim-canvas.dark-bg {
  background: #111114;
}

.sim-canvas.paper-bg {
  background: #ffffff;
}

/* ========== Right Panel ========== */
.exp-panel {
  border-left: 1px solid var(--border);
  background: var(--bg-elev);
  display: flex;
  flex-direction: column;
  overflow: hidden;
}

.panel-tabs {
  display: flex;
  border-bottom: 1px solid var(--border);
  flex-shrink: 0;
  padding: 0 10px;
  gap: 2px;
  background: #fff;
}

.panel-tab {
  flex: 1;
  padding: 14px 4px 12px;
  font-size: 13px;
  color: var(--text-tertiary);
  background: none;
  border-bottom: 2px solid transparent;
  transition: all .15s;
  font-weight: 500;
}

.panel-tab.active {
  color: var(--accent);
  border-bottom-color: var(--accent);
  font-weight: 600;
}

.panel-tab:hover:not(.active) { color: var(--text-secondary); }

.panel-content {
  flex: 1;
  overflow-y: auto;
  padding: 20px;
}

.panel-section {
  margin-bottom: 24px;
}

.panel-section-title {
  font-size: 11px;
  color: var(--text-tertiary);
  text-transform: uppercase;
  letter-spacing: 1.5px;
  margin-bottom: 12px;
  display: flex;
  align-items: center;
  gap: 8px;
  font-weight: 600;
}

.panel-section-title::before {
  content: '';
  width: 3px;
  height: 11px;
  background: var(--accent);
  border-radius: 2px;
}

/* ========== Controls ========== */
.control-group {
  margin-bottom: 16px;
}

.control-label {
  display: flex;
  justify-content: space-between;
  align-items: center;
  font-size: 13px;
  color: var(--text-secondary);
  margin-bottom: 8px;
}

.control-value {
  font-family: 'JetBrains Mono', monospace;
  font-size: 13px;
  color: var(--accent);
  font-weight: 600;
}

input[type="range"] {
  width: 100%;
  height: 4px;
  border-radius: 2px;
  background: var(--border);
  outline: none;
  -webkit-appearance: none;
  appearance: none;
}

input[type="range"]::-webkit-slider-thumb {
  -webkit-appearance: none;
  appearance: none;
  width: 16px;
  height: 16px;
  border-radius: 50%;
  background: var(--accent);
  cursor: pointer;
  box-shadow: 0 0 0 3px var(--accent-soft);
  border: 2px solid #fff;
  transition: box-shadow .15s;
}
input[type="range"]:active::-webkit-slider-thumb {
  box-shadow: 0 0 0 5px var(--accent-soft);
}

input[type="range"]::-moz-range-thumb {
  width: 14px;
  height: 14px;
  border-radius: 50%;
  background: var(--accent);
  cursor: pointer;
  border: 2px solid #fff;
}

.btn-row {
  display: flex;
  gap: 8px;
  flex-wrap: wrap;
}

.btn {
  padding: 8px 15px;
  border-radius: 8px;
  font-size: 13px;
  font-weight: 500;
  transition: all .15s;
  border: 1px solid var(--border);
  background: #fff;
  color: var(--text-secondary);
  display: inline-flex;
  align-items: center;
  gap: 6px;
}

.btn:hover {
  background: var(--bg-soft);
  border-color: var(--border-strong);
  color: var(--text-primary);
}

.btn.primary {
  background: var(--accent);
  border-color: var(--accent);
  color: #fff;
  font-weight: 600;
  box-shadow: 0 1px 3px rgba(0,0,0,0.2);
}
.btn.primary:hover { background: var(--accent-strong); border-color: var(--accent-strong); }

.btn.danger { color: var(--danger); border-color: var(--danger-soft); }
.btn.danger:hover { background: var(--danger-soft); }

.btn.sm { padding: 5px 10px; font-size: 12px; }

.btn-icon {
  width: 14px;
  height: 14px;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  flex-shrink: 0;
}

.btn-icon {
  width: 14px;
  height: 14px;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  flex-shrink: 0;
}

.btn-icon svg {
  width: 100%;
  height: 100%;
  stroke: currentColor;
  stroke-width: 2;
  fill: none;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.btn-icon svg polygon {
  fill: currentColor;
  stroke: currentColor;
}

/* Toggle Switch */
.switch {
  position: relative;
  display: inline-block;
  width: 40px;
  height: 22px;
}
.switch input { opacity: 0; width: 0; height: 0; }
.switch-slider {
  position: absolute;
  cursor: pointer;
  inset: 0;
  background: var(--border-strong);
  border-radius: 22px;
  transition: .2s;
}
.switch-slider::before {
  position: absolute;
  content: "";
  height: 16px;
  width: 16px;
  left: 3px;
  top: 3px;
  background: white;
  border-radius: 50%;
  transition: .2s;
  box-shadow: 0 1px 2px rgba(0,0,0,0.15);
}
.switch input:checked + .switch-slider {
  background: var(--accent);
}
.switch input:checked + .switch-slider::before {
  transform: translateX(18px);
}

/* ========== Meters / Gauges ========== */
.meter-row {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 10px;
  margin-bottom: 14px;
}

.meter-card {
  background: #fff;
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 14px 12px;
  text-align: center;
  transition: border-color .15s, box-shadow .15s;
}

.meter-card:hover {
  border-color: var(--border-strong);
  box-shadow: var(--shadow-sm);
}

.meter-card.full { grid-column: 1 / -1; }

.meter-label {
  font-size: 11px;
  color: var(--text-tertiary);
  margin-bottom: 4px;
  letter-spacing: 0.3px;
  font-weight: 500;
}

.meter-value {
  font-family: 'JetBrains Mono', monospace;
  font-size: 22px;
  font-weight: 600;
  color: var(--text-primary);
  line-height: 1.2;
}

.meter-unit {
  font-size: 12px;
  color: var(--text-tertiary);
  margin-left: 2px;
  font-weight: 400;
}

.meter-sub {
  font-size: 11px;
  color: var(--text-muted);
  margin-top: 4px;
  font-weight: 500;
}

/* ========== Formula Box ========== */
.formula-box {
  background: var(--accent-soft);
  border: 1px solid #e3e3e5;
  border-left: 3px solid var(--accent);
  border-radius: 8px;
  padding: 14px 16px;
  margin-bottom: 10px;
}

.formula-expr {
  font-family: 'JetBrains Mono', 'Cambria Math', monospace;
  font-size: 14px;
  color: var(--accent-text);
  margin-bottom: 10px;
  letter-spacing: 0.2px;
  font-weight: 700;
}

.formula-sub {
  font-size: 12.5px;
  color: var(--text-secondary);
  line-height: 1.8;
}

.formula-sub .val {
  color: var(--accent);
  font-family: 'JetBrains Mono', monospace;
  font-weight: 600;
}

/* ========== Chart Mini ========== */
.chart-box {
  background: #fff;
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 10px;
  height: 180px;
  margin-bottom: 10px;
}

/* ========== Explanation ========== */
.explain-box {
  background: #fff;
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 16px;
  font-size: 13px;
  color: var(--text-secondary);
  line-height: 1.7;
}

.explain-box h4 {
  font-size: 14px;
  color: var(--text-primary);
  margin-bottom: 10px;
  font-weight: 600;
}

.explain-box strong { color: var(--accent); font-weight: 600; }

.explain-box p { margin-top: 4px; }

/* ========== Select ========== */
select, input[type="number"] {
  width: 100%;
  padding: 6px 10px;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  color: var(--text-primary);
  font-size: 13px;
  outline: none;
  transition: border-color .15s;
}
select:focus, input[type="number"]:focus { border-color: var(--accent); }

/* ========== Tabs within sim area ========== */
.subtabs {
  display: flex;
  gap: 4px;
  padding: 8px 12px;
  background: var(--bg-elev);
  border-bottom: 1px solid var(--border);
  flex-shrink: 0;
}

.subtab {
  padding: 6px 14px;
  font-size: 12px;
  color: var(--text-tertiary);
  border-radius: var(--radius-sm);
  transition: all .15s;
}
.subtab.active {
  background: var(--accent);
  color: #fff;
  font-weight: 500;
}
.subtab:hover:not(.active) { color: var(--text-secondary); background: var(--bg-soft); }

/* ========== SVG icon helpers ========== */
.icon-svg {
  width: 1em;
  height: 1em;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  vertical-align: -2px;
}

.category-icon svg {
  width: 22px;
  height: 22px;
  stroke: #fff;
  stroke-width: 1.8;
  fill: none;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.app-title-icon svg {
  width: 18px;
  height: 18px;
  stroke: currentColor;
  stroke-width: 1.8;
  fill: none;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.back-btn svg {
  width: 14px;
  height: 14px;
  stroke: currentColor;
  stroke-width: 2;
  fill: none;
  stroke-linecap: round;
  stroke-linejoin: round;
}

/* Scrollbar */
.panel-content::-webkit-scrollbar { width: 6px; }
.panel-content::-webkit-scrollbar-track { background: transparent; }
.panel-content::-webkit-scrollbar-thumb { background: var(--border); border-radius: 3px; }
.portal::-webkit-scrollbar { width: 8px; }
.portal::-webkit-scrollbar-track { background: transparent; }
.portal::-webkit-scrollbar-thumb { background: var(--border); border-radius: 4px; }

/* Responsive */
@media (max-width: 900px) {
  .categories-grid { grid-template-columns: 1fr; }
  .experiment { grid-template-columns: 1fr; grid-template-rows: 1fr auto; }
  .exp-panel { border-left: none; border-top: 1px solid var(--border); max-height: 45vh; }
}

@media (max-width: 900px) {
  .hero-grid {
    grid-template-columns: 1fr;
    gap: 40px;
  }
  .hero-title { font-size: 40px !important; }
  .hero-section { padding: 48px 24px 60px; }
  .categories-section { padding: 48px 24px 72px; }
  .categories-grid { grid-template-columns: repeat(2, 1fr); }
  .section-title { font-size: 28px; }
  .experiment { grid-template-columns: 1fr; }
  .app-header { padding: 0 20px; }
  .nav-links { display: none; }
  .cat-hero { padding: 32px 24px 24px; }
  .exp-cards-grid { padding: 0 24px 40px; }
}