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/widgets/app_tab_view_builder.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import '../models/app_tab.dart';
 import '../views/activity_view.dart';
+import '../views/favorites_view.dart';
 import '../views/files_view.dart';
 import '../views/photos_view.dart';
 import '../views/recent_view.dart';
@@ -16,6 +17,8 @@ Widget buildAppTabView(AppTab tab, ScrollController controller) {
       return FilesView(scrollController: controller);
     case AppTab.photos:
       return PhotosView(scrollController: controller);
+    case AppTab.favorites:
+      return FavoritesView(scrollController: controller);
     case AppTab.activity:
       return ActivityView(scrollController: controller);
     case AppTab.trash: