:root {
  --bg: #0f1419;
  --surface: #1a2332;
  --surface-elevated: #243044;
  --border: #2d3f56;
  --text: #e8edf4;
  --text-muted: #8b9cb3;
  --accent: #3b9eff;
  --accent-hover: #5cb0ff;
  --success: #34d399;
  --success-bg: rgba(52, 211, 153, 0.12);
  --error: #f87171;
  --error-bg: rgba(248, 113, 113, 0.12);
  --warning: #fbbf24;
  --radius: 12px;
  --radius-sm: 8px;
  --shadow: 0 4px 24px rgba(0, 0, 0, 0.35);
  --font: "DM Sans", system-ui, sans-serif;
}

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

body {
  margin: 0;
  min-height: 100vh;
  font-family: var(--font);
  font-size: 16px;
  line-height: 1.5;
  color: var(--text);
  background:
    radial-gradient(ellipse 80% 50% at 50% -20%, rgba(59, 158, 255, 0.15), transparent),
    var(--bg);
}

.app {
  max-width: 720px;
  margin: 0 auto;
  padding: 1.5rem 1.25rem 3rem;
}

.app-narrow {
  max-width: 440px;
}

.app-wide {
  max-width: 960px;
}

.top-nav {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  margin-bottom: 1.5rem;
  padding: 0.75rem 1rem;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
}

.nav-brand {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  font-weight: 700;
}

.nav-user {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  flex-wrap: wrap;
  justify-content: flex-end;
}

.level-badge {
  font-size: 0.75rem;
  font-weight: 700;
  padding: 0.2rem 0.5rem;
  border-radius: 999px;
  background: rgba(59, 158, 255, 0.15);
  color: var(--accent);
}

.btn-sm {
  padding: 0.35rem 0.65rem;
  font-size: 0.8rem;
}

.header-compact {
  margin-bottom: 1rem;
}

.header-compact .tagline {
  margin-top: 0;
}

.header {
  text-align: center;
  margin-bottom: 2rem;
}

.logo {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 0.6rem;
}

.logo-icon {
  font-size: 2rem;
}

.logo h1 {
  margin: 0;
  font-size: 1.75rem;
  font-weight: 700;
  letter-spacing: -0.02em;
}

.tagline {
  margin: 0.5rem 0 0;
  color: var(--text-muted);
  font-size: 0.95rem;
}

.stats-bar {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 0.75rem;
  margin-bottom: 1.25rem;
}

.mode-tabs {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 0.5rem;
  margin-bottom: 1.25rem;
}

.mode-tab {
  padding: 0.75rem 0.65rem;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--surface);
  color: var(--text-muted);
  font-family: inherit;
  font-size: 0.85rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.15s, border-color 0.15s;
}

.mode-tab.active {
  background: rgba(59, 158, 255, 0.12);
  border-color: var(--accent);
  color: var(--accent);
}

.formula-large {
  font-size: 2rem;
  font-weight: 700;
}

.element-symbol-large,
.element-name-large {
  font-size: 2.5rem;
  font-weight: 700;
  letter-spacing: 0.02em;
}

.periodic-prompt {
  margin: 0 0 1rem;
  font-size: 1.05rem;
  color: var(--text-muted);
}

.periodic-grid-wrap {
  overflow-x: auto;
  margin: 0 -0.25rem;
  padding: 0.25rem;
}

.periodic-grid {
  display: grid;
  grid-template-columns: repeat(18, minmax(2.4rem, 1fr));
  gap: 2px;
  min-width: 36rem;
}

.periodic-cell {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  min-height: 2.75rem;
  padding: 0.15rem;
  border: 1px solid var(--border);
  border-radius: 4px;
  background: var(--surface-elevated);
  font-size: 0.6rem;
  line-height: 1.15;
  transition: border-color 0.15s, background 0.15s;
}

.periodic-cell.empty {
  border-color: transparent;
  background: transparent;
}

.periodic-cell.highlighted {
  border-color: var(--accent);
  background: rgba(59, 158, 255, 0.2);
  box-shadow: 0 0 0 1px var(--accent);
}

.periodic-cell-number {
  color: var(--text-muted);
  font-size: 0.55rem;
}

.periodic-cell-symbol {
  font-size: 0.85rem;
  font-weight: 700;
}

.periodic-cell-mass {
  color: var(--text-muted);
  font-size: 0.5rem;
}

#periodic-input {
  flex: 1;
  max-width: 240px;
  padding: 0.65rem 0.75rem;
  font-size: 1.1rem;
  font-weight: 600;
  font-family: inherit;
  border: 2px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--bg);
  color: var(--text);
}

#periodic-input:focus {
  outline: none;
  border-color: var(--accent);
}

#periodic-input.correct {
  border-color: var(--success);
  background: var(--success-bg);
}

#periodic-input.incorrect {
  border-color: var(--error);
  background: var(--error-bg);
}

#periodic-input.hint-revealed {
  border-color: var(--warning);
  background: rgba(251, 191, 36, 0.1);
}

.molmass-form {
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
  margin-bottom: 1rem;
}

.molmass-form label {
  font-size: 0.875rem;
  color: var(--text-muted);
}

.molmass-input-row {
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

.molmass-input-row input {
  flex: 1;
  max-width: 200px;
  padding: 0.65rem 0.75rem;
  font-size: 1.2rem;
  font-weight: 600;
  font-family: inherit;
  border: 2px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--bg);
  color: var(--text);
}

.molmass-input-row input:focus {
  outline: none;
  border-color: var(--accent);
}

.molmass-input-row input.correct {
  border-color: var(--success);
  background: var(--success-bg);
}

.molmass-input-row input.incorrect {
  border-color: var(--error);
  background: var(--error-bg);
}

.molmass-input-row input.hint-revealed {
  border-color: var(--warning);
  background: rgba(251, 191, 36, 0.1);
}

.molmass-unit {
  color: var(--text-muted);
  font-size: 0.9rem;
}

.already-logged-in {
  text-align: center;
}

.already-logged-in p {
  margin: 0 0 1rem;
}

.login-back-link {
  margin: 1.25rem 0 0;
  text-align: center;
  font-size: 0.875rem;
}

.login-back-link a {
  color: var(--text-muted);
  text-decoration: none;
}

.login-back-link a:hover {
  color: var(--accent);
}

.btn-check {
  align-self: flex-start;
}

.atom-table .total-row td {
  border-top: 2px solid var(--border);
  padding-top: 0.75rem;
}

.stat {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  padding: 0.75rem 1rem;
  text-align: center;
}

.stat-label {
  display: block;
  font-size: 0.75rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--text-muted);
}

.stat-value {
  display: block;
  font-size: 1.5rem;
  font-weight: 700;
  color: var(--accent);
  margin-top: 0.15rem;
}

.controls {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 0.75rem;
  margin-bottom: 1.25rem;
}

.controls label {
  font-size: 0.875rem;
  color: var(--text-muted);
}

.controls select {
  flex: 1;
  min-width: 180px;
  padding: 0.55rem 0.75rem;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--surface);
  color: var(--text);
  font-family: inherit;
  font-size: 0.9rem;
  cursor: pointer;
}

.controls select:focus {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}

.btn {
  padding: 0.55rem 1rem;
  border: none;
  border-radius: var(--radius-sm);
  font-family: inherit;
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.15s, transform 0.1s;
}

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

.btn-primary {
  background: var(--accent);
  color: #fff;
}

.btn-primary:hover {
  background: var(--accent-hover);
}

.btn-secondary {
  background: var(--surface-elevated);
  color: var(--text);
  border: 1px solid var(--border);
}

.btn-secondary:hover {
  background: var(--border);
}

.btn-ghost {
  background: transparent;
  color: var(--text-muted);
  border: 1px solid var(--border);
}

.btn-ghost:hover {
  color: var(--text);
  border-color: var(--text-muted);
}

.equation-card,
.atom-table-card,
.help-card,
.leaderboard-card,
.join-class-card,
.xp-card,
.auth-card,
.class-card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.5rem;
  margin-bottom: 1.25rem;
  box-shadow: var(--shadow);
}

.card-label {
  margin: 0 0 1rem;
  font-size: 0.8rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--text-muted);
}

.equation {
  font-size: 1.35rem;
  text-align: center;
  padding: 1.25rem 0.5rem;
  margin-bottom: 1.25rem;
  background: var(--surface-elevated);
  border-radius: var(--radius-sm);
  line-height: 1.8;
  overflow-x: auto;
}

.equation .compound {
  display: inline-flex;
  align-items: baseline;
  margin: 0 0.15rem;
}

.equation .coeff {
  font-weight: 700;
  color: var(--accent);
  min-width: 1.2em;
  text-align: right;
}

.equation .coeff.empty {
  color: transparent;
  user-select: none;
}

.equation .formula sub {
  font-size: 0.75em;
  vertical-align: sub;
}

.equation .plus,
.equation .arrow {
  margin: 0 0.5rem;
  color: var(--text-muted);
}

.equation .arrow {
  font-weight: 600;
}

.balance-form {
  margin-top: 1rem;
}

.coefficients {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 0.75rem 1.25rem;
  margin-bottom: 1.25rem;
}

.coeff-group {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 0.35rem;
}

.coeff-group label {
  font-size: 0.8rem;
  color: var(--text-muted);
}

.coeff-group input {
  width: 3.5rem;
  padding: 0.5rem;
  text-align: center;
  font-size: 1.1rem;
  font-weight: 600;
  font-family: inherit;
  border: 2px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--bg);
  color: var(--text);
}

.coeff-group input:focus {
  outline: none;
  border-color: var(--accent);
}

.coeff-group input.correct {
  border-color: var(--success);
  background: var(--success-bg);
}

.coeff-group input.incorrect {
  border-color: var(--error);
  background: var(--error-bg);
}

.coeff-group input.hint-revealed {
  border-color: var(--warning);
  background: rgba(251, 191, 36, 0.1);
}

.form-actions {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 0.5rem;
}

.feedback {
  margin-top: 1.25rem;
  padding: 1rem 1.25rem;
  border-radius: var(--radius-sm);
  font-weight: 500;
  text-align: center;
}

.feedback.hidden {
  display: none;
}

.feedback.success {
  background: var(--success-bg);
  color: var(--success);
  border: 1px solid rgba(52, 211, 153, 0.3);
}

.feedback.error {
  background: var(--error-bg);
  color: var(--error);
  border: 1px solid rgba(248, 113, 113, 0.3);
}

.feedback.info {
  background: rgba(59, 158, 255, 0.12);
  color: var(--accent);
  border: 1px solid rgba(59, 158, 255, 0.3);
}

.hidden {
  display: none !important;
}

.text-muted {
  color: var(--text-muted);
}

/* XP & gamification */
.xp-card {
  padding: 1rem 1.25rem;
}

.xp-header {
  display: flex;
  justify-content: space-between;
  align-items: baseline;
  margin-bottom: 0.5rem;
}

.xp-label {
  font-size: 0.75rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--text-muted);
}

#xp-total {
  font-size: 1.25rem;
  font-weight: 700;
  color: var(--accent);
}

.xp-bar-track {
  height: 8px;
  background: var(--surface-elevated);
  border-radius: 999px;
  overflow: hidden;
}

.xp-bar-fill {
  height: 100%;
  width: 0%;
  background: linear-gradient(90deg, var(--accent), #7cc4ff);
  border-radius: 999px;
  transition: width 0.4s ease;
}

.xp-sub {
  margin: 0.5rem 0 0;
  font-size: 0.8rem;
  color: var(--text-muted);
}

/* Leaderboard */
.leaderboard-card h2,
.join-class-card h2 {
  margin: 0 0 0.5rem;
  font-size: 1.1rem;
}

.leaderboard-list {
  list-style: none;
  margin: 0;
  padding: 0;
}

.leaderboard-item {
  display: grid;
  grid-template-columns: 2rem 1fr auto auto;
  align-items: center;
  gap: 0.75rem;
  padding: 0.75rem 0;
  border-bottom: 1px solid var(--border);
}

.leaderboard-item:last-child {
  border-bottom: none;
}

.leaderboard-medal {
  font-size: 1.25rem;
  text-align: center;
}

.leaderboard-name {
  font-weight: 600;
}

.leaderboard-xp {
  font-weight: 700;
  color: var(--accent);
}

.leaderboard-meta {
  font-size: 0.8rem;
  color: var(--text-muted);
}

.leaderboard-item.rank-1 {
  background: rgba(251, 191, 36, 0.06);
  border-radius: var(--radius-sm);
  padding-left: 0.5rem;
  padding-right: 0.5rem;
}

/* Join class */
.join-class-form {
  display: flex;
  gap: 0.5rem;
  flex-wrap: wrap;
}

.join-class-form input {
  flex: 1;
  min-width: 140px;
  padding: 0.55rem 0.75rem;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--bg);
  color: var(--text);
  font-family: inherit;
  font-size: 0.9rem;
}

/* Auth */
.auth-tabs {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 0.5rem;
  margin-bottom: 1.25rem;
}

.auth-tab {
  padding: 0.6rem;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--surface-elevated);
  color: var(--text-muted);
  font-family: inherit;
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
}

.auth-tab.active {
  background: rgba(59, 158, 255, 0.15);
  border-color: var(--accent);
  color: var(--accent);
}

.auth-form {
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

.form-field label {
  display: block;
  margin-bottom: 0.35rem;
  font-size: 0.875rem;
  color: var(--text-muted);
}

.form-field input[type="email"],
.form-field input[type="password"],
.form-field input[type="text"] {
  width: 100%;
  padding: 0.6rem 0.75rem;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--bg);
  color: var(--text);
  font-family: inherit;
  font-size: 0.95rem;
}

.form-field input:focus {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}

.field-hint {
  margin: 0.35rem 0 0;
  font-size: 0.8rem;
  color: var(--text-muted);
}

.role-picker {
  display: flex;
  gap: 0.75rem;
}

.role-option {
  flex: 1;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 0.4rem;
  padding: 0.65rem;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--bg);
  cursor: pointer;
  font-size: 0.9rem;
}

.role-option:has(input:checked) {
  border-color: var(--accent);
  background: rgba(59, 158, 255, 0.1);
}

.btn-full {
  width: 100%;
}

.btn:disabled {
  opacity: 0.6;
  cursor: not-allowed;
}

/* Teacher dashboard */
.dashboard-header {
  display: flex;
  flex-wrap: wrap;
  align-items: flex-end;
  justify-content: space-between;
  gap: 1rem;
  margin-bottom: 1.25rem;
}

.dashboard-header h1 {
  margin: 0 0 0.25rem;
  font-size: 1.5rem;
}

.create-class-form {
  display: flex;
  gap: 0.5rem;
  flex-wrap: wrap;
}

.create-class-form input {
  min-width: 200px;
  padding: 0.55rem 0.75rem;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--surface);
  color: var(--text);
  font-family: inherit;
}

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

.class-card-header h2 {
  margin: 0 0 0.25rem;
  font-size: 1.15rem;
}

.join-code {
  margin: 0;
  font-size: 0.9rem;
  color: var(--text-muted);
}

.join-code strong {
  color: var(--accent);
  letter-spacing: 0.08em;
}

.student-count {
  font-size: 0.8rem;
  color: var(--text-muted);
  white-space: nowrap;
}

.teacher-table td:first-child {
  text-align: left;
}

.empty-state,
.empty-class,
.loading-text {
  text-align: center;
  color: var(--text-muted);
  padding: 2rem 1rem;
}

.atom-table-card h2,
.help-card h2 {
  margin: 0 0 0.5rem;
  font-size: 1.1rem;
}

.table-hint,
.help-card ol {
  margin: 0 0 1rem;
  font-size: 0.875rem;
  color: var(--text-muted);
}

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

.atom-table {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.9rem;
}

.atom-table th,
.atom-table td {
  padding: 0.6rem 0.75rem;
  text-align: center;
  border-bottom: 1px solid var(--border);
}

.atom-table th {
  font-size: 0.75rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--text-muted);
  font-weight: 600;
}

.atom-table td:first-child {
  font-weight: 600;
  text-align: left;
}

.atom-table .balanced {
  color: var(--success);
}

.atom-table .unbalanced {
  color: var(--error);
}

.help-card ol {
  padding-left: 1.25rem;
  margin-bottom: 0;
}

.help-card li {
  margin-bottom: 0.4rem;
}

.footer {
  text-align: center;
  padding-top: 1rem;
}

.footer p {
  margin: 0;
  font-size: 0.8rem;
  color: var(--text-muted);
}

@media (max-width: 480px) {
  .equation {
    font-size: 1.1rem;
  }

  .stats-bar {
    gap: 0.5rem;
  }

  .stat-value {
    font-size: 1.25rem;
  }
}