Add dark theme + auto accent color overrides

Use off-white backgrounds with dark text for active/selected elements
when using dark theme with 'auto' accent color:
- Tab buttons (active state)
- Sidebar nav items (active state)
- Primary buttons
- Toggle switches
- Badges, checkboxes, input focus states
- Theme settings cards and options

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/styles/Layout.css

@@ -428,4 +428,21 @@
 .tab-content-placeholder p {
   margin: 0;
   font-size: 1rem;
+}
+
+/* ==========  DARK THEME + AUTO ACCENT OVERRIDES ========== */
+
+/* Tab buttons with auto accent in dark mode: use off-white background with dark text */
+[data-theme='dark'][data-accent='auto'] .page-tab-btn.active,
+[data-theme='dark'][data-accent='auto'] .admin-tab-btn.active {
+  background: #e5e7eb;
+  color: #111827;
+  box-shadow: 0 2px 8px rgba(229, 231, 235, 0.2);
+}
+
+[data-theme='dark'][data-accent='auto'] .page-tab-btn.active:focus,
+[data-theme='dark'][data-accent='auto'] .admin-tab-btn.active:focus,
+[data-theme='dark'][data-accent='auto'] .page-tab-btn.active:focus-visible,
+[data-theme='dark'][data-accent='auto'] .admin-tab-btn.active:focus-visible {
+  box-shadow: 0 2px 8px rgba(229, 231, 235, 0.3);
 }