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/application/dtos/folder_dto.rs

@@ -52,6 +52,17 @@ pub struct FolderDto {
 
     /// Whether this is a root folder
     pub is_root: bool,
+
+    // ── Pre-computed display fields ──
+
+    /// FontAwesome icon CSS class (always "fas fa-folder")
+    pub icon_class: String,
+
+    /// Extra CSS class for icon styling (always "folder-icon")
+    pub icon_special_class: String,
+
+    /// Human-readable category (always "Folder")
+    pub category: String,
 }
 
 impl From<Folder> for FolderDto {
@@ -67,6 +78,9 @@ impl From<Folder> for FolderDto {
             created_at: folder.created_at(),
             modified_at: folder.modified_at(),
             is_root,
+            icon_class: "fas fa-folder".to_string(),
+            icon_special_class: "folder-icon".to_string(),
+            category: "Folder".to_string(),
         }
     }
 }
@@ -74,8 +88,8 @@ impl From<Folder> for FolderDto {
 // To convert from FolderDto to Folder for batch handlers
 impl From<FolderDto> for Folder {
     fn from(dto: FolderDto) -> Self {
-        // Use constructor to create an entity from DTO
-        // Note: this should be simplified if Folder has a proper constructor
+        // Display fields (icon_class, icon_special_class, category)
+        // are not part of the domain entity and are ignored.
         Folder::from_dto(
             dto.id,
             dto.name,
@@ -99,6 +113,9 @@ impl FolderDto {
             created_at: 0,
             modified_at: 0,
             is_root: true,
+            icon_class: "fas fa-folder".to_string(),
+            icon_special_class: "folder-icon".to_string(),
+            category: "Folder".to_string(),
         }
     }
 }