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/models/app_tab.dart

@@ -4,7 +4,7 @@ import 'package:material_symbols_icons/symbols.dart';
 /// Every destination the bottom nav bar can show. Order here is only the
 /// fallback default — actual order/visibility/default-tab are user
 /// configurable and persisted in [ServerProvider].
-enum AppTab { files, photos, activity, trash, shares, recent }
+enum AppTab { files, photos, favorites, activity, trash, shares, recent }
 
 /// At most this many tabs may be visible in the bottom nav bar at once —
 /// the rest are reachable through the "more" dropdown instead.
@@ -17,6 +17,8 @@ extension AppTabInfo on AppTab {
         return 'Files';
       case AppTab.photos:
         return 'Photos';
+      case AppTab.favorites:
+        return 'Favorites';
       case AppTab.activity:
         return 'Activity';
       case AppTab.trash:
@@ -34,6 +36,8 @@ extension AppTabInfo on AppTab {
         return Icons.folder_rounded;
       case AppTab.photos:
         return Icons.photo_library_rounded;
+      case AppTab.favorites:
+        return Icons.favorite_rounded;
       case AppTab.activity:
         return Icons.electric_bolt_rounded;
       case AppTab.trash: