Add dark theme support and bookmark status to share view

- Implement dark/light theme toggle with persistent preference via
  localStorage
- Add a sticky toolbar with theme toggle, Markdown export, and PDF
  export buttons
- Update bookmark button to reflect current state with visual feedback
- Introduce CSS custom properties for theming and responsive layout
  improvements

frontend/style.css

@@ -5830,6 +5830,15 @@ body.popup-mode .content-area {
 .search-replace-btn.destructive { border-color: var(--text-error); color: var(--text-error); }
 
 /* ── Active search result highlight ── */
+.btn-bookmark.active {
+  color: #059669;
+  border-color: #059669;
+  background: rgba(5, 150, 105, 0.08);
+}
+.btn-bookmark.active:hover {
+  background: rgba(5, 150, 105, 0.15);
+}
+
 .btn-share.shared {
   color: #059669;
   border-color: #059669;