Add in-app copy/move, background downloads, and a Favorites tab

- WebDAV MOVE/COPY-backed copy/move for files and folders, with a
  destination picker (independent nav state so it doesn't disturb the
  Files tab's browsing position) and a conflict-resolution sheet
  (overwrite all / keep both / decide per item).
- Downloads now use the same foreground-service + notification approach
  as uploads, surviving app closure.
- Favorites is now its own tab (account-wide, via a WebDAV SEARCH query)
  instead of a Files-tab filter toggle, which couldn't represent
  favorited items outside the currently browsed folder correctly.
- Share sheet restyled to match the app's design language, plus a
  "share file directly" action via the OS share sheet; details sheet
  tab icons resized to match the bottom nav.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/views/share_upload_view.dart

@@ -108,15 +108,6 @@ class _ShareUploadViewState extends State<ShareUploadView> {
                 onChanged: provider.setFilesSortField,
               ),
             ),
-            ToggleIconButton(
-              icon: provider.showFavoritesOnlyFiles
-                  ? Icons.favorite_rounded
-                  : Icons.favorite_border_rounded,
-              isSelected: provider.showFavoritesOnlyFiles,
-              onTap: provider.toggleFavoritesFilterFiles,
-              tooltip: 'Favorites only',
-            ),
-            const SizedBox(width: 4),
             ToggleIconButton(
               icon: provider.showHiddenFiles
                   ? Icons.visibility_rounded