/* 
  Calculus Calculator - Main Stylesheet
  Design: Modern Minimalist, Responsive, Dark/Light Mode
*/

:root {
  /* Light Theme Variables */
  --bg-color: #f8fafc;
  --bg-surface: #ffffff;
  --bg-surface-hover: #f1f5f9;
  --text-primary: #0f172a;
  --text-secondary: #475569;
  --border-color: #e2e8f0;
  
  --primary-color: #6366f1; /* Indigo */
  --primary-hover: #4f46e5;
  --secondary-color: #8b5cf6; /* Violet */
  --accent-gradient: linear-gradient(135deg, var(--primary-color), var(--secondary-color));
  
  --shadow-sm: 0 1px 2px 0 rgb(0 0 0 / 0.05);
  --shadow-md: 0 4px 6px -1px rgb(0 0 0 / 0.1), 0 2px 4px -2px rgb(0 0 0 / 0.1);
  --shadow-lg: 0 10px 15px -3px rgb(0 0 0 / 0.1), 0 4px 6px -4px rgb(0 0 0 / 0.1);
  
  --radius-md: 0.5rem;
  --radius-lg: 0.75rem;
  --radius-xl: 1rem;
  
  --success: #10b981;
  --error: #ef4444;
  --warning: #f59e0b;
}

[data-theme="dark"] {
  /* Dark Theme Variables */
  --bg-color: #0f172a;
  --bg-surface: #1e293b;
  --bg-surface-hover: #334155;
  --text-primary: #f8fafc;
  --text-secondary: #94a3b8;
  --border-color: #334155;
  
  --primary-color: #818cf8;
  --primary-hover: #6366f1;
  --secondary-color: #a78bfa;
  
  --shadow-sm: 0 1px 2px 0 rgb(0 0 0 / 0.5);
  --shadow-md: 0 4px 6px -1px rgb(0 0 0 / 0.4);
  --shadow-lg: 0 10px 15px -3px rgb(0 0 0 / 0.4);
}

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

body {
  font-family: 'Inter', system-ui, -apple-system, sans-serif;
  background-color: var(--bg-color);
  color: var(--text-primary);
  line-height: 1.6;
  transition: background-color 0.3s ease, color 0.3s ease;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

/* --- Layout --- */
.container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 1.5rem;
  width: 100%;
}

header {
  background-color: var(--bg-surface);
  border-bottom: 1px solid var(--border-color);
  padding: 1rem 0;
  position: sticky;
  top: 0;
  z-index: 50;
  transition: background-color 0.3s ease;
}

.header-content {
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.logo {
  font-size: 1.5rem;
  font-weight: 700;
  background: var(--accent-gradient);
  -webkit-background-clip: text;
  background-clip: text;
  -webkit-text-fill-color: transparent;
  text-decoration: none;
}

.nav-links {
  display: flex;
  gap: 1.5rem;
  align-items: center;
}

.nav-links a {
  color: var(--text-secondary);
  text-decoration: none;
  font-weight: 500;
  transition: color 0.2s;
}

.nav-links a:hover, .nav-links a.active {
  color: var(--primary-color);
}

.theme-toggle {
  background: none;
  border: none;
  color: var(--text-secondary);
  cursor: pointer;
  font-size: 1.25rem;
  padding: 0.5rem;
  border-radius: var(--radius-md);
  transition: background-color 0.2s;
}

.theme-toggle:hover {
  background-color: var(--bg-surface-hover);
}

/* --- Auth UI --- */
.auth-btn {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  background-color: var(--bg-surface);
  border: 1px solid var(--border-color);
  color: var(--text-primary);
  padding: 0.5rem 1rem;
  border-radius: var(--radius-md);
  font-weight: 600;
  font-size: 0.9rem;
  cursor: pointer;
  transition: all 0.2s;
}

.auth-btn:hover {
  background-color: var(--bg-surface-hover);
  border-color: var(--primary-color);
}

.google-icon {
  display: inline-grid;
  place-items: center;
  width: 18px;
  height: 18px;
  border-radius: 50%;
  background: #ffffff;
  color: #2563eb;
  font-size: 0.78rem;
  font-weight: 900;
  line-height: 1;
}

#auth-authenticated {
  display: flex;
  align-items: center;
  gap: 0.75rem;
}

.user-profile {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  position: relative;
  cursor: pointer;
  border: none;
  background: var(--bg-surface-hover);
  padding: 0.35rem 0.8rem 0.35rem 0.35rem;
  border-radius: var(--radius-md);
  box-shadow: var(--shadow-sm);
}

.user-avatar {
  width: 32px;
  height: 32px;
  border-radius: 50%;
  object-fit: cover;
  border: 2px solid var(--primary-color);
}

.user-info {
  display: none;
}

@media (min-width: 640px) {
  .user-info {
    display: flex;
    flex-direction: column;
    gap: 0.2rem;
    font-size: 0.85rem;
    font-weight: 600;
    line-height: 1.2;
  }
}

.user-name-row {
  display: flex;
  align-items: center;
  gap: 0.45rem;
  color: var(--text-primary);
  white-space: nowrap;
}

.user-plan-badge {
  display: none;
  padding: 0.18rem 0.55rem;
  border-radius: 999px;
  font-size: 0.68rem;
  font-weight: 800;
  letter-spacing: 0.04em;
  text-transform: uppercase;
  color: #ffffff;
}

.user-center-hint {
  color: var(--text-secondary);
  font-size: 0.74rem;
  font-weight: 500;
}

.logout-link {
  color: var(--error) !important;
  font-size: 0.8rem !important;
  white-space: nowrap;
}

.user-center-overlay {
  position: fixed;
  inset: 0;
  background: rgba(15, 23, 42, 0.48);
  backdrop-filter: blur(3px);
  z-index: 80;
}

.user-center-panel {
  position: fixed;
  top: 0;
  right: 0;
  width: min(440px, 100%);
  height: 100vh;
  padding: 1.5rem;
  background: var(--bg-surface);
  border-left: 1px solid var(--border-color);
  box-shadow: var(--shadow-lg);
  overflow-y: auto;
  z-index: 90;
}

.user-center-header {
  display: flex;
  justify-content: space-between;
  align-items: flex-start;
  gap: 1rem;
  margin-bottom: 1.5rem;
}

.user-center-eyebrow {
  color: var(--primary-color);
  text-transform: uppercase;
  letter-spacing: 0.08em;
  font-size: 0.76rem;
  font-weight: 800;
  margin-bottom: 0.35rem;
}

.user-center-close {
  border: none;
  background: var(--bg-color);
  color: var(--text-secondary);
  width: 40px;
  height: 40px;
  border-radius: 999px;
  font-size: 1.7rem;
  cursor: pointer;
}

.user-center-profile {
  display: flex;
  align-items: center;
  gap: 1rem;
  margin-bottom: 1.5rem;
}

.user-center-avatar {
  width: 56px;
  height: 56px;
  border-radius: 50%;
  object-fit: cover;
  border: 3px solid var(--primary-color);
}

.user-center-email {
  color: var(--text-secondary);
  font-size: 0.92rem;
}

.user-center-section {
  margin-bottom: 1.5rem;
}

.user-center-section h3 {
  font-size: 1rem;
  margin-bottom: 0.8rem;
}

.user-center-card {
  background: var(--bg-color);
  border: 1px solid var(--border-color);
  border-radius: 1rem;
  padding: 1rem;
}

.plan-summary-card {
  padding: 1.2rem;
}

.plan-summary-top {
  display: flex;
  justify-content: space-between;
  align-items: flex-start;
  gap: 1rem;
  margin-bottom: 1rem;
}

.user-center-plan-badge {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-width: 74px;
  padding: 0.35rem 0.75rem;
  border-radius: 999px;
  font-size: 0.78rem;
  font-weight: 800;
  text-transform: uppercase;
  color: #ffffff;
  background: linear-gradient(135deg, #64748b, #475569);
}

.user-center-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 0.9rem;
}

.user-center-grid.single {
  grid-template-columns: 1fr;
}

.user-center-label {
  color: var(--text-secondary);
  font-size: 0.78rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  font-weight: 700;
  margin-bottom: 0.35rem;
}

.user-center-value {
  font-size: 1.5rem;
  font-weight: 800;
}

.user-center-actions {
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
}

.user-center-action {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 100%;
  min-height: 46px;
  border: none;
  border-radius: 999px;
  background: linear-gradient(135deg, #2563eb, #1d4ed8);
  color: #ffffff !important;
  text-decoration: none;
  font-weight: 700;
  cursor: pointer;
}

.user-center-action.danger {
  background: linear-gradient(135deg, #dc2626, #b91c1c);
}

.user-center-action:disabled {
  opacity: 0.5;
  cursor: not-allowed;
}

.user-center-note {
  margin-top: 0.85rem;
  color: var(--text-secondary);
  font-size: 0.9rem;
}

.auth-hidden {
  display: none !important;
}

main {
  flex: 1;
  padding: 2rem 0;
}

/* --- Calculator Interface --- */
.calculator-layout {
  display: grid;
  grid-template-columns: 1fr;
  gap: 2rem;
}

@media (min-width: 1024px) {
  .calculator-layout {
    grid-template-columns: 400px 1fr;
  }
}

/* Left Panel: Input */
.input-panel {
  background-color: var(--bg-surface);
  border-radius: var(--radius-xl);
  padding: 1.5rem;
  box-shadow: var(--shadow-md);
  height: fit-content;
}

.tabs {
  display: flex;
  flex-wrap: wrap;
  background-color: var(--bg-color);
  border-radius: var(--radius-lg);
  padding: 0.25rem;
  margin-bottom: 1.5rem;
  gap: 0.25rem;
}

.tab-btn {
  flex: 1;
  padding: 0.75rem 1rem;
  border: none;
  background: transparent;
  color: var(--text-secondary);
  font-weight: 600;
  font-size: 0.95rem;
  cursor: pointer;
  border-radius: var(--radius-md);
  transition: all 0.2s;
}

.tab-btn.active {
  background-color: var(--bg-surface);
  color: var(--primary-color);
  box-shadow: var(--shadow-sm);
}

.input-group {
  margin-bottom: 1.5rem;
}

.input-group label {
  display: block;
  font-weight: 600;
  margin-bottom: 0.5rem;
  color: var(--text-primary);
  font-size: 0.95rem;
}

.math-input-wrapper {
  position: relative;
}

.math-input {
  width: 100%;
  padding: 1rem;
  font-size: 1.1rem;
  font-family: monospace;
  border: 2px solid var(--border-color);
  border-radius: var(--radius-lg);
  background-color: var(--bg-color);
  color: var(--text-primary);
  transition: border-color 0.2s;
}

.math-input:focus {
  outline: none;
  border-color: var(--primary-color);
}

.katex-preview {
  margin-top: 0.75rem;
  min-height: 3rem;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 1rem;
  background-color: var(--bg-color);
  border-radius: var(--radius-lg);
  color: var(--text-primary);
  font-size: 1.2rem;
  overflow-x: auto;
  border: 1px dashed var(--border-color);
}

.options-row {
  display: flex;
  gap: 1rem;
  margin-bottom: 1.5rem;
}

.options-row > div {
  flex: 1;
}

.select-input, .bounds-input {
  width: 100%;
  padding: 0.75rem;
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
  background-color: var(--bg-color);
  color: var(--text-primary);
  font-size: 0.95rem;
}

.bounds-input {
  text-align: center;
}

.btn-primary {
  width: 100%;
  padding: 1rem;
  background: var(--accent-gradient);
  color: white;
  border: none;
  border-radius: var(--radius-lg);
  font-size: 1.1rem;
  font-weight: 600;
  cursor: pointer;
  transition: transform 0.1s, opacity 0.2s;
  display: flex;
  justify-content: center;
  align-items: center;
  gap: 0.5rem;
}

.btn-primary:hover {
  opacity: 0.9;
}

.btn-primary:active {
  transform: scale(0.98);
}

.btn-primary:disabled {
  opacity: 0.7;
  cursor: not-allowed;
  transform: none;
}

/* Loading Spinner */
.spinner {
  width: 20px;
  height: 20px;
  border: 3px solid rgba(255,255,255,0.3);
  border-radius: 50%;
  border-top-color: white;
  animation: spin 1s linear infinite;
  display: none;
}
@keyframes spin { 100% { transform: rotate(360deg); } }

.examples {
  margin-top: 1.5rem;
  font-size: 0.85rem;
}

.examples p {
  color: var(--text-secondary);
  margin-bottom: 0.5rem;
}

.example-tags {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
}

.example-tag {
  background-color: var(--bg-color);
  border: 1px solid var(--border-color);
  color: var(--text-secondary);
  padding: 0.3rem 0.6rem;
  border-radius: 1rem;
  cursor: pointer;
  transition: all 0.2s;
}

.example-tag:hover {
  border-color: var(--primary-color);
  color: var(--primary-color);
}

/* Right Panel: Results */
.result-panel {
  display: flex;
  flex-direction: column;
  gap: 1.5rem;
}

.result-card {
  background-color: var(--bg-surface);
  border-radius: var(--radius-xl);
  padding: 1.5rem;
  box-shadow: var(--shadow-md);
}

.result-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 1.5rem;
  border-bottom: 1px solid var(--border-color);
  padding-bottom: 0.75rem;
}

.result-header h2 {
  font-size: 1.25rem;
  color: var(--text-primary);
}

.copy-btn {
  background: none;
  border: none;
  color: var(--text-secondary);
  cursor: pointer;
  font-size: 0.9rem;
  display: flex;
  align-items: center;
  gap: 0.4rem;
}

.copy-btn:hover {
  color: var(--primary-color);
}

.final-result {
  font-size: 1.5rem;
  padding: 1.5rem;
  background-color: var(--bg-color);
  border-radius: var(--radius-lg);
  overflow-x: auto;
  text-align: center;
  border: 2px solid var(--primary-color);
}

/* Steps */
.steps-container {
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

.step-card {
  background-color: var(--bg-color);
  border-radius: var(--radius-lg);
  padding: 1.25rem;
  border-left: 4px solid var(--primary-color);
}

.step-card.nested {
  margin-left: 2rem;
  border-left-color: var(--secondary-color);
  background-color: transparent;
  border: 1px solid var(--border-color);
  border-left: 4px solid var(--secondary-color);
}

.step-rule {
  display: inline-block;
  background-color: var(--bg-surface);
  color: var(--primary-color);
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  padding: 0.2rem 0.5rem;
  border-radius: 0.25rem;
  margin-bottom: 0.5rem;
  border: 1px solid var(--border-color);
}

.step-desc {
  color: var(--text-secondary);
  margin-bottom: 0.75rem;
  font-size: 0.95rem;
}

.step-math {
  font-size: 1.1rem;
  overflow-x: auto;
  padding: 0.5rem 0;
}

/* Graph */
.graph-container {
  width: 100%;
  height: 400px;
  background-color: var(--bg-color);
  border-radius: var(--radius-lg);
  overflow: hidden;
}

/* Alerts & Errors */
.alert {
  padding: 1rem;
  border-radius: var(--radius-md);
  margin-bottom: 1rem;
  display: flex;
  flex-direction: column;
  gap: 0.5rem;
}

.alert-error {
  background-color: rgba(239, 68, 68, 0.1);
  border: 1px solid var(--error);
  color: var(--error);
}

.alert-title {
  font-weight: 700;
}

.alert-suggestion {
  font-size: 0.9rem;
  opacity: 0.9;
}

.hidden {
  display: none !important;
}

/* --- Mobile Symbol Keyboard --- */
.symbol-keyboard {
  background-color: var(--bg-surface);
  border-top: 1px solid var(--border-color);
  padding: 0.5rem;
  display: grid;
  grid-template-columns: repeat(7, 1fr);
  gap: 0.25rem;
}

@media (min-width: 1024px) {
  .symbol-keyboard {
    display: none; /* Hide on regular desktop unless activated */
  }
}

.key-btn {
  background-color: var(--bg-color);
  border: 1px solid var(--border-color);
  color: var(--text-primary);
  border-radius: 0.25rem;
  padding: 0.5rem 0;
  font-size: 1rem;
  font-weight: 500;
  cursor: pointer;
  touch-action: manipulation;
}

.key-btn:active {
  background-color: var(--border-color);
}

.key-btn.action {
  background-color: rgba(99, 102, 241, 0.1);
  color: var(--primary-color);
  font-weight: 700;
}

/* Footer */
footer {
  text-align: center;
  padding: 2rem 0;
  color: var(--text-secondary);
  font-size: 0.9rem;
  border-top: 1px solid var(--border-color);
  background-color: var(--bg-surface);
}

.footer-nav {
  display: flex;
  justify-content: center;
  flex-wrap: wrap;
  gap: 1.25rem;
  margin-bottom: 0.9rem;
}

.footer-nav a {
  color: var(--text-secondary);
  text-decoration: none;
  font-weight: 600;
}

.footer-nav a:hover {
  color: var(--primary-color);
}

.support-notice {
  font-size: 0.85rem;
  margin-top: 1rem;
  padding: 1rem;
  background-color: var(--bg-color);
  border-radius: var(--radius-md);
  color: var(--text-secondary);
}

/* --- Pricing Section --- */
.pricing-section {
  margin-top: 4rem;
  padding: 3rem 0 1rem;
}

.pricing-hero {
  position: relative;
  overflow: hidden;
  background:
    radial-gradient(circle at top left, rgba(99, 102, 241, 0.18), transparent 35%),
    radial-gradient(circle at top right, rgba(139, 92, 246, 0.18), transparent 30%),
    linear-gradient(180deg, var(--bg-surface) 0%, var(--bg-color) 100%);
  border: 1px solid var(--border-color);
  border-radius: 2rem;
  padding: 2.5rem;
  box-shadow: var(--shadow-lg);
  margin-bottom: 2rem;
}

.pricing-hero::after {
  content: "";
  position: absolute;
  inset: auto -40px -60px auto;
  width: 180px;
  height: 180px;
  background: radial-gradient(circle, rgba(250, 204, 21, 0.22), transparent 70%);
  pointer-events: none;
}

.pricing-copy {
  max-width: 700px;
}

.pricing-eyebrow {
  display: inline-block;
  margin-bottom: 0.85rem;
  padding: 0.35rem 0.8rem;
  border-radius: 999px;
  background: rgba(99, 102, 241, 0.12);
  color: var(--primary-color);
  font-size: 0.85rem;
  font-weight: 700;
  letter-spacing: 0.04em;
  text-transform: uppercase;
}

.pricing-copy h2 {
  font-size: clamp(2rem, 4vw, 3rem);
  line-height: 1.15;
  margin-bottom: 0.9rem;
}

.pricing-copy p {
  color: var(--text-secondary);
  font-size: 1.05rem;
  max-width: 640px;
}

.billing-toggle {
  margin-top: 2rem;
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  padding: 0.5rem;
  border-radius: 999px;
  background: rgba(148, 163, 184, 0.14);
  box-shadow: var(--shadow-md);
}

.billing-option {
  border: none;
  background: transparent;
  color: var(--text-secondary);
  border-radius: 999px;
  padding: 0.95rem 1.4rem;
  font-size: 1rem;
  font-weight: 700;
  cursor: pointer;
  transition: all 0.2s ease;
}

.billing-option.active {
  background: var(--bg-surface);
  color: var(--text-primary);
  box-shadow: var(--shadow-sm);
}

.billing-badge {
  display: inline-flex;
  align-items: center;
  margin-left: 0.5rem;
  padding: 0.2rem 0.55rem;
  border-radius: 999px;
  background: #fbbf24;
  color: #442c00;
  font-size: 0.82rem;
}

.pricing-cards {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 1.5rem;
  align-items: stretch;
}

.pricing-card {
  position: relative;
  display: flex;
  flex-direction: column;
  gap: 1.25rem;
  background: var(--bg-surface);
  border: 1px solid var(--border-color);
  border-radius: 2rem;
  padding: 1.9rem;
  box-shadow: var(--shadow-md);
}

.pricing-card.featured {
  border: 2px solid #fbbf24;
  box-shadow: 0 24px 50px -24px rgba(251, 191, 36, 0.55), var(--shadow-lg);
}

.plan-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 0.75rem;
}

.plan-header h3 {
  font-size: 2rem;
  line-height: 1.1;
}

.plan-tag {
  display: inline-flex;
  align-items: center;
  padding: 0.35rem 0.8rem;
  border-radius: 999px;
  background: rgba(99, 102, 241, 0.12);
  color: var(--primary-color);
  font-size: 0.82rem;
  font-weight: 700;
}

.plan-tag.subtle {
  background: rgba(148, 163, 184, 0.12);
  color: var(--text-secondary);
}

.plan-tag.highlight {
  background: #fbbf24;
  color: #442c00;
}

.plan-summary {
  min-height: 3.5rem;
  color: var(--text-secondary);
  font-size: 1.02rem;
}

.plan-price {
  display: flex;
  align-items: flex-end;
  gap: 0.6rem;
}

.price-value {
  font-size: clamp(2.5rem, 4vw, 3.4rem);
  line-height: 1;
  font-weight: 800;
  letter-spacing: -0.04em;
}

.price-period {
  color: var(--text-secondary);
  font-size: 1.1rem;
  padding-bottom: 0.35rem;
}

.plan-billing-note {
  color: var(--text-secondary);
  font-size: 0.92rem;
  min-height: 1.5rem;
}

.plan-button {
  width: 100%;
  border: none;
  border-radius: 999px;
  padding: 1rem 1.25rem;
  font-size: 1.05rem;
  font-weight: 700;
  color: #ffffff;
  background: linear-gradient(135deg, #2563eb, #1d4ed8);
  cursor: pointer;
  transition: transform 0.15s ease, opacity 0.2s ease, box-shadow 0.2s ease;
  box-shadow: 0 18px 30px -22px rgba(37, 99, 235, 0.85);
}

.plan-button:hover {
  opacity: 0.95;
  transform: translateY(-1px);
}

.plan-button.ghost {
  color: var(--text-primary);
  background: rgba(148, 163, 184, 0.12);
  box-shadow: none;
}

.plan-features {
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: 0.8rem;
  padding-top: 1.4rem;
  border-top: 1px solid var(--border-color);
}

.plan-features li {
  position: relative;
  padding-left: 1.6rem;
  color: var(--text-secondary);
}

.plan-features li::before {
  content: "•";
  position: absolute;
  left: 0.2rem;
  color: var(--primary-color);
  font-size: 1.1rem;
  line-height: 1;
}

.pricing-compare {
  margin-top: 2rem;
  background: var(--bg-surface);
  border: 1px solid var(--border-color);
  border-radius: 2rem;
  padding: 2rem;
  box-shadow: var(--shadow-md);
}

.payment-support-note {
  margin-top: 1.25rem;
  padding: 1rem 1.25rem;
  border: 1px dashed var(--border-color);
  border-radius: 1rem;
  background: var(--bg-surface);
  color: var(--text-secondary);
  text-align: center;
}

.about-section {
  margin-top: 4rem;
  padding-bottom: 2rem;
}

.about-shell {
  background:
    radial-gradient(circle at top right, rgba(99, 102, 241, 0.14), transparent 30%),
    linear-gradient(180deg, var(--bg-surface) 0%, var(--bg-color) 100%);
  border: 1px solid var(--border-color);
  border-radius: 2rem;
  padding: 2.25rem;
  box-shadow: var(--shadow-md);
}

.about-intro {
  max-width: 760px;
  margin-bottom: 1.5rem;
}

.about-eyebrow {
  display: inline-block;
  margin-bottom: 0.85rem;
  padding: 0.35rem 0.8rem;
  border-radius: 999px;
  background: rgba(99, 102, 241, 0.12);
  color: var(--primary-color);
  font-size: 0.85rem;
  font-weight: 700;
  letter-spacing: 0.04em;
  text-transform: uppercase;
}

.about-intro h2 {
  font-size: clamp(1.8rem, 3.8vw, 2.7rem);
  line-height: 1.15;
  margin-bottom: 0.85rem;
}

.about-intro p {
  color: var(--text-secondary);
  font-size: 1.02rem;
}

.about-grid {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 1rem;
}

.about-card {
  background: var(--bg-surface);
  border: 1px solid var(--border-color);
  border-radius: 1.25rem;
  padding: 1.25rem;
}

.about-card h3 {
  margin-bottom: 0.75rem;
}

.about-card ul {
  padding-left: 1.15rem;
  color: var(--text-secondary);
}

.about-card li + li {
  margin-top: 0.45rem;
}

.compare-header {
  margin-bottom: 1.25rem;
}

.compare-header h3 {
  font-size: 1.5rem;
  margin-bottom: 0.4rem;
}

.compare-header p {
  color: var(--text-secondary);
}

.pricing-table-wrap {
  overflow-x: auto;
}

.pricing-table {
  width: 100%;
  border-collapse: collapse;
}

.pricing-table th,
.pricing-table td {
  padding: 1rem 1rem;
  border-bottom: 1px solid var(--border-color);
  text-align: left;
}

.pricing-table th {
  color: var(--text-primary);
  font-size: 0.95rem;
}

.pricing-table td {
  color: var(--text-secondary);
  font-size: 0.96rem;
}

.pricing-table tbody tr:last-child td {
  border-bottom: none;
}

@media (max-width: 1023px) {
  .pricing-cards {
    grid-template-columns: 1fr;
  }

  .about-grid {
    grid-template-columns: 1fr;
  }

  .plan-summary {
    min-height: auto;
  }
}

@media (max-width: 767px) {
  .nav-links {
    gap: 0.8rem;
  }

  #auth-authenticated {
    gap: 0.45rem;
  }

  .user-profile {
    padding-right: 0.5rem;
  }

  .user-center-panel {
    width: 100%;
    padding: 1.1rem;
  }

  .user-center-grid {
    grid-template-columns: 1fr;
  }

  .pricing-section {
    margin-top: 3rem;
    padding-top: 2rem;
  }

  .pricing-hero,
  .pricing-card,
  .pricing-compare {
    border-radius: 1.5rem;
  }

  .pricing-hero {
    padding: 1.5rem;
  }

  .billing-toggle {
    width: 100%;
    justify-content: space-between;
  }

  .billing-option {
    flex: 1;
    padding: 0.85rem 1rem;
  }

  .billing-badge {
    display: block;
    margin: 0.35rem 0 0;
  }

  .pricing-compare {
    padding: 1.25rem;
  }

  .pricing-table th,
  .pricing-table td {
    min-width: 140px;
    padding: 0.85rem 0.75rem;
  }
}

/* --- Homepage Refresh --- */
:root {
  --brand-navy: #0d1b2a;
  --brand-teal: #00c9a7;
  --brand-teal-deep: #00a88a;
  --brand-amber: #f5a623;
  --brand-blue: #3b82f6;
  --glass-bg: rgba(255, 255, 255, 0.72);
  --soft-border: rgba(15, 23, 42, 0.1);
}

[data-theme="dark"] {
  --glass-bg: rgba(13, 27, 42, 0.78);
  --soft-border: rgba(255, 255, 255, 0.1);
}

html {
  scroll-behavior: smooth;
}

body {
  background:
    radial-gradient(circle at 16% 8%, rgba(0, 201, 167, 0.14), transparent 32rem),
    radial-gradient(circle at 84% 24%, rgba(245, 166, 35, 0.12), transparent 28rem),
    linear-gradient(var(--bg-color), var(--bg-color));
  position: relative;
  overflow-x: hidden;
}

body::before {
  content: "";
  position: fixed;
  inset: 0;
  background-image:
    linear-gradient(rgba(0, 201, 167, 0.045) 1px, transparent 1px),
    linear-gradient(90deg, rgba(59, 130, 246, 0.04) 1px, transparent 1px);
  background-size: 48px 48px;
  pointer-events: none;
  z-index: -1;
}

header {
  background: var(--glass-bg);
  backdrop-filter: blur(18px);
  border-bottom-color: var(--soft-border);
  padding: 0.8rem 0;
}

.logo {
  display: inline-flex;
  align-items: center;
  gap: 0.7rem;
  background: none;
  -webkit-text-fill-color: currentColor;
  color: var(--text-primary);
}

.logo-mark {
  display: inline-grid;
  place-items: center;
  width: 38px;
  height: 38px;
  border-radius: 0.6rem;
  background: linear-gradient(135deg, var(--brand-teal), #7dd3fc);
  color: #06231f;
  font-size: 1.45rem;
  font-weight: 800;
  box-shadow: 0 14px 30px -18px rgba(0, 201, 167, 0.9);
}

.logo-text {
  font-size: 1.05rem;
  font-weight: 800;
}

.site-main {
  flex: 1;
  padding: 0 0 2rem;
}

.hero-section {
  position: relative;
  min-height: min(760px, calc(100vh - 66px));
  display: flex;
  align-items: center;
  padding: 5.5rem 0 4rem;
  overflow: hidden;
}

.hero-glow {
  position: absolute;
  width: 34rem;
  height: 34rem;
  border-radius: 50%;
  filter: blur(8px);
  pointer-events: none;
}

.hero-glow-a {
  top: -13rem;
  left: 8%;
  background: radial-gradient(circle, rgba(0, 201, 167, 0.16), transparent 68%);
}

.hero-glow-b {
  right: -10rem;
  bottom: 0;
  background: radial-gradient(circle, rgba(245, 166, 35, 0.15), transparent 68%);
}

.hero-grid {
  position: relative;
  z-index: 1;
  display: grid;
  grid-template-columns: minmax(0, 1.02fr) minmax(360px, 0.88fr);
  gap: 4rem;
  align-items: center;
}

.hero-copy {
  max-width: 720px;
}

.hero-badge {
  display: inline-flex;
  align-items: center;
  gap: 0.55rem;
  margin-bottom: 1.3rem;
  padding: 0.42rem 0.8rem;
  border: 1px solid rgba(0, 201, 167, 0.34);
  border-radius: 999px;
  background: rgba(0, 201, 167, 0.1);
  color: var(--brand-teal);
  font-size: 0.78rem;
  font-weight: 800;
  letter-spacing: 0.06em;
  text-transform: uppercase;
}

.hero-badge span {
  width: 0.48rem;
  height: 0.48rem;
  border-radius: 50%;
  background: var(--brand-teal);
  box-shadow: 0 0 0 0 rgba(0, 201, 167, 0.45);
  animation: badgePulse 2.2s ease-out infinite;
}

@keyframes badgePulse {
  70% { box-shadow: 0 0 0 10px rgba(0, 201, 167, 0); }
  100% { box-shadow: 0 0 0 0 rgba(0, 201, 167, 0); }
}

.hero-copy h1 {
  max-width: 760px;
  margin-bottom: 1.25rem;
  font-size: clamp(2.75rem, 6vw, 5.7rem);
  line-height: 0.98;
  letter-spacing: 0;
}

.hero-copy p {
  max-width: 640px;
  color: var(--text-secondary);
  font-size: 1.12rem;
}

.hero-actions {
  display: flex;
  flex-wrap: wrap;
  gap: 0.85rem;
  margin: 2rem 0;
}

.hero-cta {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-height: 48px;
  padding: 0.88rem 1.25rem;
  border-radius: var(--radius-sm, 0.5rem);
  text-decoration: none;
  font-weight: 800;
  transition: transform 0.18s ease, border-color 0.18s ease, background 0.18s ease;
}

.hero-cta.primary {
  background: var(--brand-teal);
  color: #06231f;
  box-shadow: 0 18px 32px -22px rgba(0, 201, 167, 1);
}

.hero-cta.secondary {
  border: 1px solid var(--soft-border);
  color: var(--text-primary);
  background: rgba(255, 255, 255, 0.05);
}

.hero-cta:hover {
  transform: translateY(-2px);
}

.hero-stats {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  max-width: 560px;
  border-top: 1px solid var(--soft-border);
  padding-top: 1.2rem;
  gap: 1rem;
}

.hero-stats div + div {
  border-left: 1px solid var(--soft-border);
  padding-left: 1rem;
}

.hero-stats strong {
  display: block;
  color: var(--text-primary);
  font-size: 1.5rem;
  line-height: 1.15;
}

.hero-stats span {
  display: block;
  color: var(--text-secondary);
  font-size: 0.82rem;
}

.hero-preview-card {
  background: rgba(15, 23, 42, 0.86);
  color: #f8fafc;
  border: 1px solid rgba(255, 255, 255, 0.11);
  border-radius: 0.9rem;
  padding: 1rem;
  box-shadow: 0 28px 90px -50px rgba(0, 0, 0, 0.9);
  transform: translateY(0);
  transition: transform 0.25s ease, border-color 0.25s ease;
}

.hero-preview-card:hover {
  transform: translateY(-4px);
  border-color: rgba(0, 201, 167, 0.36);
}

.preview-topbar,
.preview-graph-head {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
}

.preview-topbar {
  padding-bottom: 0.85rem;
  border-bottom: 1px solid rgba(255, 255, 255, 0.1);
}

.preview-topbar > div {
  display: inline-flex;
  align-items: center;
  gap: 0.55rem;
}

.preview-dot {
  width: 0.65rem;
  height: 0.65rem;
  border-radius: 50%;
  background: var(--brand-teal);
}

.preview-title,
.preview-graph-head > span {
  font-size: 0.82rem;
  font-weight: 800;
}

.preview-pill {
  padding: 0.25rem 0.55rem;
  border: 1px solid rgba(0, 201, 167, 0.34);
  border-radius: 999px;
  color: var(--brand-teal);
  font-size: 0.74rem;
  font-weight: 800;
}

.preview-input,
.preview-result {
  margin-top: 1rem;
  border: 1px solid rgba(255, 255, 255, 0.09);
  border-radius: 0.6rem;
  background: rgba(255, 255, 255, 0.045);
  padding: 0.85rem;
}

.preview-input span,
.preview-result span {
  display: block;
  margin-bottom: 0.35rem;
  color: #8ba3bb;
  font-size: 0.76rem;
  font-weight: 800;
  text-transform: uppercase;
}

.preview-input strong,
.preview-result div {
  font-family: ui-monospace, SFMono-Regular, Consolas, monospace;
  font-size: 1rem;
}

.preview-result div {
  color: var(--brand-teal);
}

.preview-steps {
  display: grid;
  gap: 0.65rem;
  margin: 1rem 0;
}

.preview-step {
  display: grid;
  grid-template-columns: 2.4rem 1fr;
  gap: 0.75rem;
  align-items: start;
}

.preview-step > span {
  display: inline-grid;
  place-items: center;
  width: 2.2rem;
  height: 2.2rem;
  border-radius: 50%;
  background: rgba(0, 201, 167, 0.12);
  border: 1px solid rgba(0, 201, 167, 0.3);
  color: var(--brand-teal);
  font-size: 0.75rem;
  font-weight: 900;
}

.preview-step strong {
  display: block;
  color: #ffffff;
  font-size: 0.88rem;
}

.preview-step p {
  margin-top: 0.1rem;
  color: #8ba3bb;
  font-size: 0.8rem;
}

#heroGraphCanvas {
  display: block;
  width: 100%;
  height: auto;
  margin-top: 0.8rem;
  border-radius: 0.55rem;
  background:
    linear-gradient(rgba(255, 255, 255, 0.045) 1px, transparent 1px),
    linear-gradient(90deg, rgba(255, 255, 255, 0.045) 1px, transparent 1px),
    rgba(0, 0, 0, 0.18);
  background-size: 28px 28px;
}

.calculator-section {
  padding: 1rem 0 0;
}

.input-panel,
.result-card,
.pricing-hero,
.pricing-card,
.pricing-compare,
.about-shell {
  border: 1px solid var(--soft-border);
  transition: transform 0.18s ease, border-color 0.18s ease, box-shadow 0.18s ease;
}

.input-panel:hover,
.result-card:hover,
.pricing-card:hover,
.about-card:hover {
  border-color: rgba(0, 201, 167, 0.3);
}

.btn-primary {
  background: linear-gradient(135deg, var(--brand-teal), #38bdf8);
  color: #06231f;
}

.tab-btn.active {
  color: var(--brand-teal);
}

.math-input:focus,
.select-input:focus,
.bounds-input:focus {
  outline: none;
  border-color: var(--brand-teal);
  box-shadow: 0 0 0 3px rgba(0, 201, 167, 0.12);
}

.final-result {
  border-color: rgba(0, 201, 167, 0.55);
}

.steps-timeline {
  position: relative;
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

.steps-timeline::before {
  content: "";
  position: absolute;
  top: 1.6rem;
  bottom: 1.6rem;
  left: 1.25rem;
  width: 2px;
  background: linear-gradient(var(--brand-teal), rgba(59, 130, 246, 0.35));
}

.step-card {
  position: relative;
  display: grid;
  grid-template-columns: 2.7rem 1fr;
  gap: 1rem;
  border-left: 0;
  border: 1px solid var(--soft-border);
  background:
    linear-gradient(180deg, rgba(0, 201, 167, 0.05), transparent),
    var(--bg-color);
}

.step-card.nested {
  margin-left: 0;
  grid-template-columns: 2.7rem 1fr;
  background: rgba(148, 163, 184, 0.06);
  border-left: 0;
}

.step-index {
  position: relative;
  z-index: 1;
  display: inline-grid;
  place-items: center;
  width: 2.5rem;
  height: 2.5rem;
  border-radius: 50%;
  background: var(--bg-surface);
  border: 1px solid rgba(0, 201, 167, 0.4);
  color: var(--brand-teal);
  font-size: 0.78rem;
  font-weight: 900;
  box-shadow: var(--shadow-sm);
}

.step-content {
  min-width: 0;
}

.step-meta {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
  margin-bottom: 0.55rem;
}

.step-rule {
  background: rgba(0, 201, 167, 0.1);
  border-color: rgba(0, 201, 167, 0.25);
  color: var(--brand-teal);
  border-radius: 999px;
}

.step-math {
  margin-top: 0.7rem;
  padding: 0.85rem;
  border: 1px solid var(--soft-border);
  border-radius: 0.6rem;
  background: var(--bg-surface);
}

.sub-steps-container {
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
  margin-top: 1rem;
}

.graph-result-header {
  align-items: flex-start;
}

.graph-result-header p {
  margin-top: 0.2rem;
  color: var(--text-secondary);
  font-size: 0.9rem;
}

.graph-legend {
  display: inline-flex;
  flex-wrap: wrap;
  gap: 0.8rem;
  align-items: center;
  color: var(--text-secondary);
  font-size: 0.84rem;
}

.graph-legend.compact {
  color: #8ba3bb;
  gap: 0.6rem;
}

.graph-legend span {
  display: inline-flex;
  align-items: center;
  gap: 0.35rem;
  white-space: nowrap;
}

.graph-legend i {
  width: 0.65rem;
  height: 0.65rem;
  border-radius: 50%;
}

.legend-f {
  background: var(--brand-teal);
}

.legend-d {
  background: var(--brand-amber);
}

.graph-container {
  border: 1px solid var(--soft-border);
  background:
    linear-gradient(var(--bg-color), var(--bg-color)) padding-box,
    linear-gradient(135deg, rgba(0, 201, 167, 0.3), rgba(245, 166, 35, 0.18)) border-box;
}

.pricing-card-lite {
  z-index: 1;
  border: 2px solid var(--brand-teal);
  background:
    radial-gradient(circle at top right, rgba(0, 201, 167, 0.14), transparent 38%),
    var(--bg-surface);
  box-shadow: 0 26px 60px -34px rgba(0, 201, 167, 0.85), var(--shadow-lg);
  transform: translateY(-0.75rem);
}

.pricing-card-pro {
  border: 1px solid rgba(245, 166, 35, 0.42);
  background:
    radial-gradient(circle at top right, rgba(245, 166, 35, 0.13), transparent 40%),
    var(--bg-surface);
}

.pricing-card-pro .plan-header h3,
.pricing-card-pro .price-value {
  color: var(--brand-amber);
}

.pro-tag {
  background: rgba(245, 166, 35, 0.14);
  color: var(--brand-amber);
}

.pricing-card-lite .plan-button {
  background: linear-gradient(135deg, var(--brand-teal), var(--brand-teal-deep));
  color: #06231f;
}

.pricing-card-pro .plan-button {
  background: linear-gradient(135deg, var(--brand-amber), #d97706);
  color: #241400;
}

.plan-features li::before {
  content: "";
  top: 0.45rem;
  width: 0.55rem;
  height: 0.55rem;
  border-radius: 50%;
  background: var(--brand-teal);
}

.pricing-card-pro .plan-features li::before {
  background: var(--brand-amber);
}

.trust-strip {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 0.65rem;
  margin-bottom: 0.8rem;
}

.trust-strip span {
  display: inline-flex;
  align-items: center;
  min-height: 2rem;
  padding: 0.35rem 0.7rem;
  border: 1px solid var(--soft-border);
  border-radius: 999px;
  background: rgba(148, 163, 184, 0.08);
  color: var(--text-secondary);
  font-size: 0.84rem;
  font-weight: 700;
}

@media (max-width: 1023px) {
  .hero-section {
    min-height: auto;
    padding: 4rem 0 2rem;
  }

  .hero-grid {
    grid-template-columns: 1fr;
    gap: 2rem;
  }

  .hero-preview-card {
    max-width: 640px;
  }

  .pricing-card-lite {
    transform: none;
  }
}

@media (max-width: 767px) {
  header .container {
    padding: 0 1rem;
  }

  .header-content {
    align-items: flex-start;
    gap: 0.75rem;
  }

  .nav-links {
    justify-content: flex-end;
    flex-wrap: wrap;
  }

  .logo-text {
    display: none;
  }

  .hero-section {
    padding-top: 2.5rem;
  }

  .hero-copy h1 {
    font-size: clamp(2.35rem, 14vw, 3.45rem);
  }

  .hero-stats {
    grid-template-columns: 1fr;
  }

  .hero-stats div + div {
    border-left: 0;
    padding-left: 0;
  }

  .hero-actions {
    flex-direction: column;
  }

  .hero-cta {
    width: 100%;
  }

  .hero-preview-card {
    padding: 0.8rem;
  }

  .calculator-layout {
    gap: 1.25rem;
  }

  .input-panel,
  .result-card {
    padding: 1rem;
  }

  .steps-timeline::before {
    left: 1rem;
  }

  .step-card,
  .step-card.nested {
    grid-template-columns: 2.2rem 1fr;
    gap: 0.75rem;
    padding: 1rem;
  }

  .step-index {
    width: 2rem;
    height: 2rem;
    font-size: 0.68rem;
  }

  .graph-result-header {
    flex-direction: column;
  }
}