Add bookmark support and enhance share dialog

- Add bookmark button to file header and context menu
- Implement toggle bookmark API call with toast notification
- Redesign share dialog to show existing shares with revocation
- Add expiration options when creating a new share
- Add CSS styles for share action buttons

frontend/style.css

@@ -5744,6 +5744,24 @@ body.popup-mode .content-area {
   cursor: pointer;
   font-size: 0.85rem;
 }
+.share-create-btn {
+  padding: 6px 16px;
+  background: var(--accent);
+  color: #fff;
+  border: none;
+  border-radius: 6px;
+  cursor: pointer;
+  font-size: 0.85rem;
+}
+.share-revoke-btn {
+  padding: 6px 16px;
+  background: none;
+  color: var(--text-error);
+  border: 1px solid var(--text-error);
+  border-radius: 6px;
+  cursor: pointer;
+  font-size: 0.85rem;
+}
 
 /* ── Enhanced Search Bar Toggles ── */
 .search-actions .tog {