Add workspace background customization and fix launcher drawer drag

- Add workspace background image support with file-based storage
  - Store background images in $APPLOCALDATA/files directory
  - Save file paths in database (text column in haex_workspaces)
  - Use convertFileSrc for secure asset:// URL conversion
  - Add context menu to workspaces with "Hintergrund ändern" option

- Implement background management in settings
  - File selection dialog for PNG, JPG, JPEG, WebP images
  - Copy selected images to app data directory
  - Remove background with file cleanup
  - Multilingual UI (German/English)

- Fix launcher drawer drag interference
  - Add :handle-only="true" to UDrawer to restrict drag to handle
  - Simplify drag handlers (removed complex state tracking)
  - Items can now be dragged to desktop without drawer interference

- Extend Tauri asset protocol scope to include $APPLOCALDATA/**
  for background image loading

src-tauri/tauri.conf.json

@@ -64,7 +64,7 @@
       },
       "assetProtocol": {
         "enable": true,
-        "scope": ["$APPDATA", "$RESOURCE"]
+        "scope": ["$APPDATA", "$RESOURCE", "$APPLOCALDATA/**"]
       }
     }
   },