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/move_copy_result.dart

@@ -0,0 +1,33 @@
+import 'nextcloud_item.dart';
+
+/// A resolution choice for one conflicting item in a Move/Copy batch -
+/// keyed by `item.id` in [ConflictChoice] maps passed to
+/// `ServerProvider.resolveConflicts`.
+enum ConflictChoice { overwrite, keepBoth, skip }
+
+/// One item that couldn't be moved/copied because something with the same
+/// name already exists at the destination (a WebDAV 412 response).
+class MoveCopyConflict {
+  final NextcloudItem item;
+
+  const MoveCopyConflict(this.item);
+}
+
+/// Outcome of a Move/Copy batch (an initial attempt via
+/// `ServerProvider.moveItems`/`copyItems`, or a follow-up
+/// `resolveConflicts` call). [blockedReason] is set instead of attempting
+/// anything at all when the destination itself is invalid (e.g. moving a
+/// folder into its own subfolder).
+class MoveCopyResult {
+  final int succeeded;
+  final List<MoveCopyConflict> conflicts;
+  final int failed;
+  final String? blockedReason;
+
+  const MoveCopyResult({
+    this.succeeded = 0,
+    this.conflicts = const [],
+    this.failed = 0,
+    this.blockedReason,
+  });
+}