feat(P1+P2): server-authoritative favorites/recent + pre-computed display fields

P1-A: Enrich FavoriteItemDto & RecentItemDto with item_name, item_size,
      item_mime_type, parent_id, modified_at via SQL LEFT JOINs — eliminates
      N+1 per-item fetches.

P1-B: Rewrite favorites.js as server-authoritative (724→380 lines).
      In-memory cache backed by GET /api/favorites; no localStorage.

P1-C: Rewrite recent.js as server-authoritative (341→249 lines).
      GET /api/recent + POST /api/recent/{type}/{id}; no localStorage.

P1 cleanup: Remove dead localStorage cleanup from auth.js logout,
            fix async clearRecentFiles in app.js.

P2: Add icon_class, icon_special_class, category, size_formatted to
    FileDto, FolderDto, FavoriteItemDto, RecentItemDto. New shared
    display_helpers.rs module centralises mime→icon/category/size logic.
    Frontend (ui.js, fileRenderer.js, favorites.js, recent.js) now reads
    pre-computed fields from the API with fallback defaults — eliminates
    5 duplicated mime→icon mapping blocks (~89 lines removed).

Also fixes: synthetic FolderDto in webdav_handler.rs, pre-existing
            missing item_name field in share_service.rs test.

Net: -307 lines across 15 files. cargo check: 0 errors, 0 warnings.
     cargo test display_helpers: 3/3 pass.

src/interfaces/api/handlers/webdav_handler.rs

@@ -216,6 +216,9 @@ async fn handle_propfind(
             created_at: Utc::now().timestamp() as u64,
             modified_at: Utc::now().timestamp() as u64,
             is_root: true,
+            icon_class: "fas fa-folder".to_string(),
+            icon_special_class: "folder-icon".to_string(),
+            category: "Folder".to_string(),
         };
 
         // Generate response