/* ── App Layout ── */

.app {
  display: grid;
  grid-template-columns: var(--sidebar-width) 1fr;
  height: 100vh;
  overflow: hidden;
}

/* ── Sidebar ── */

.sidebar {
  background: var(--sidebar-bg);
  display: flex;
  flex-direction: column;
  overflow: hidden;
  border-right: 1px solid rgba(255, 255, 255, 0.06);
}

.sidebar-brand {
  padding: var(--sp-6) var(--sp-5);
  border-bottom: 1px solid var(--sidebar-border);
  flex-shrink: 0;
}

.sidebar-brand h1 {
  font-size: var(--text-lg);
  font-weight: 700;
  color: var(--sidebar-text-active);
  letter-spacing: -0.3px;
  line-height: 1.2;
}

.sidebar-brand p {
  font-size: var(--text-sm);
  color: var(--sidebar-text);
  margin-top: 2px;
}

/* Progress bar */
.sidebar-progress {
  padding: var(--sp-4) var(--sp-5);
  border-bottom: 1px solid var(--sidebar-border);
  flex-shrink: 0;
}

.progress-label {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: var(--sp-2);
}

.progress-label span {
  font-size: var(--text-xs);
  color: var(--sidebar-text);
  font-weight: 500;
  text-transform: uppercase;
  letter-spacing: 0.5px;
}

.progress-label strong {
  font-size: var(--text-sm);
  color: var(--sidebar-text-active);
  font-weight: 600;
}

.progress-track {
  height: 4px;
  background: var(--sidebar-hover);
  border-radius: 2px;
  overflow: hidden;
}

.progress-fill {
  height: 100%;
  background: var(--success);
  border-radius: 2px;
  transition: width var(--transition-slow);
  width: 0%;
}

/* Navigation list */
.nav-list {
  flex: 1;
  overflow-y: auto;
  padding: var(--sp-3) 0;
  scrollbar-width: thin;
  scrollbar-color: var(--sidebar-hover) transparent;
}

/* Section groups */
.nav-section {
  margin-bottom: var(--sp-1);
}

.nav-section-header {
  display: flex;
  align-items: center;
  gap: var(--sp-2);
  padding: var(--sp-2) var(--sp-5);
  cursor: pointer;
  user-select: none;
  transition: background var(--transition-fast);
}

.nav-section-header:hover {
  background: rgba(255, 255, 255, 0.03);
}

.nav-section-chevron {
  font-size: 10px;
  color: var(--sidebar-section);
  transition: transform var(--transition-fast);
  width: 12px;
  text-align: center;
}

.nav-section.collapsed .nav-section-chevron {
  transform: rotate(-90deg);
}

.nav-section-title {
  font-size: var(--text-xs);
  font-weight: 600;
  color: var(--sidebar-section);
  text-transform: uppercase;
  letter-spacing: 0.8px;
}

.nav-section-items {
  overflow: hidden;
  transition: max-height var(--transition-slow);
}

.nav-section.collapsed .nav-section-items {
  max-height: 0 !important;
}

/* Nav items */
.nav-item {
  display: flex;
  align-items: center;
  gap: var(--sp-3);
  padding: var(--sp-2) var(--sp-5);
  padding-left: var(--sp-8);
  cursor: pointer;
  transition: all var(--transition-fast);
  border-left: 3px solid transparent;
  position: relative;
}

.nav-item:hover {
  background: var(--sidebar-hover);
}

.nav-item.active {
  background: rgba(37, 99, 235, 0.12);
  border-left-color: var(--primary);
}

.nav-item.active .nav-name {
  color: var(--sidebar-text-active);
  font-weight: 600;
}

.nav-item.active .nav-num {
  background: var(--primary);
  color: white;
}

.nav-num {
  width: 22px;
  height: 22px;
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: var(--text-xs);
  font-weight: 600;
  color: var(--sidebar-text);
  background: var(--sidebar-hover);
  flex-shrink: 0;
  transition: all var(--transition-fast);
}

.nav-item.done .nav-num {
  background: var(--success);
  color: white;
}

.nav-item.done .nav-num::after {
  content: "\2713";
  font-size: 11px;
}

.nav-item.done .nav-num span {
  display: none;
}

.nav-name {
  font-size: var(--text-sm);
  color: var(--sidebar-text);
  transition: color var(--transition-fast);
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

/* Sidebar footer */
.sidebar-footer {
  padding: var(--sp-3) var(--sp-5);
  border-top: 1px solid var(--sidebar-border);
  flex-shrink: 0;
  display: flex;
  flex-direction: column;
  gap: var(--sp-2);
}

.reset-progress-btn {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 6px;
  width: 100%;
  padding: var(--sp-2) var(--sp-3);
  font-size: var(--text-xs);
  font-family: var(--font);
  font-weight: 500;
  color: var(--sidebar-text);
  background: var(--sidebar-hover);
  border: 1px solid rgba(255, 255, 255, 0.08);
  border-radius: var(--radius-sm);
  cursor: pointer;
  transition: all var(--transition-fast);
}

.reset-progress-btn:hover {
  background: rgba(255, 255, 255, 0.08);
  color: var(--sidebar-text-active);
}

.sidebar-link {
  display: block;
  text-align: center;
  font-size: var(--text-xs);
  color: var(--sidebar-section);
  text-decoration: none;
  transition: color var(--transition-fast);
}

.sidebar-link:hover {
  color: var(--primary);
}

/* ── Main Content ── */

.main-content {
  display: flex;
  flex-direction: column;
  overflow: hidden;
  background: var(--bg);
}

/* Title bar */
.title-bar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: var(--sp-3) var(--sp-6);
  background: var(--surface);
  border-bottom: 1px solid var(--border);
  height: var(--titlebar-height);
  flex-shrink: 0;
}

.title-text h2 {
  font-size: var(--text-xl);
  font-weight: 700;
  color: var(--text);
  letter-spacing: -0.3px;
}

.title-text p {
  font-size: var(--text-sm);
  color: var(--text-secondary);
  margin-top: 1px;
}

.title-meta {
  display: flex;
  align-items: center;
  gap: var(--sp-3);
}

.concept-counter {
  font-size: var(--text-sm);
  color: var(--text-muted);
  font-weight: 500;
  font-variant-numeric: tabular-nums;
}

/* ── Context Bar (Excel explanation) ── */

.context-bar {
  display: flex;
  align-items: flex-start;
  gap: var(--sp-2);
  padding: var(--sp-2) var(--sp-6);
  background: var(--excel-light);
  border-bottom: 1px solid var(--border);
  flex-shrink: 0;
  min-height: 32px;
}

.context-icon {
  color: var(--excel);
  font-size: 12px;
  margin-top: 2px;
  flex-shrink: 0;
}

.context-bar span {
  font-size: var(--text-sm);
  color: var(--text-secondary);
  line-height: 1.4;
}

/* ── Tab Bar ── */

.tab-bar {
  display: flex;
  align-items: center;
  gap: var(--sp-1);
  padding: 0 var(--sp-6);
  background: var(--surface);
  border-bottom: 1px solid var(--border);
  height: var(--tabbar-height);
  flex-shrink: 0;
}

.tab-btn {
  display: flex;
  align-items: center;
  gap: 6px;
  padding: var(--sp-2) var(--sp-4);
  font-size: var(--text-sm);
  font-weight: 500;
  font-family: var(--font);
  color: var(--text-secondary);
  background: none;
  border: none;
  border-bottom: 2px solid transparent;
  cursor: pointer;
  transition: all var(--transition-fast);
  height: 100%;
}

.tab-btn:hover {
  color: var(--text);
  background: var(--border-light);
}

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

.tab-btn i {
  font-size: 12px;
}

/* ── Instruction Banner (above panels) ── */

.instruction-banner {
  display: flex;
  align-items: center;
  gap: var(--sp-3);
  padding: var(--sp-3) var(--sp-6);
  margin: 0 var(--sp-3);
  background: var(--primary-light);
  border-left: 3px solid var(--primary);
  border-radius: 0;
  flex-shrink: 0;
  min-height: 40px;
}

.instruction-banner.challenge-mode {
  background: var(--warning-light);
  border-left-color: var(--warning);
}

.instruction-badge {
  display: inline-flex;
  align-items: center;
  gap: 5px;
  padding: 2px 10px;
  font-size: var(--text-xs);
  font-weight: 700;
  color: var(--primary);
  background: white;
  border-radius: 100px;
  letter-spacing: 0.3px;
  flex-shrink: 0;
  white-space: nowrap;
}

.instruction-banner.challenge-mode .instruction-badge {
  color: var(--warning);
}

.instruction-badge i {
  font-size: 10px;
}

.instruction-text {
  flex: 1;
  font-size: var(--text-sm);
  color: var(--text);
  line-height: 1.4;
}

.instruction-text code {
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  color: var(--primary-dark);
  background: rgba(255, 255, 255, 0.7);
  padding: 1px 5px;
  border-radius: 3px;
}

.instruction-actions {
  flex-shrink: 0;
}

/* ── Split Panels ── */

.panels {
  flex: 1;
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 1px;
  background: var(--border);
  overflow: hidden;
  margin: 0 var(--sp-3) var(--sp-3);
  border-radius: 0 0 var(--radius-md) var(--radius-md);
}