Add device sync: mirror folders/files locally, background WorkManager engine

- New native sync engine (SyncEngine/SyncWorker/ConflictResolveWorker,
  WorkManager-based) that mirrors selected Nextcloud folders or files to
  app-private local storage, periodically and on-demand ("Sync now"),
  with never-auto-resolved conflict notifications.
- Single files (not just folders) can now be marked "Sync to device".
- Live sync status (syncing/synced/conflicts) pushes from native to Dart
  over a new EventChannel; the persistent header chip/panel now reflects
  device-sync status instead of the WebDAV-refresh loading state, with a
  cloud_off/cloud_sync/cloud_done/cloud_alert icon set and an expandable
  conflicts list with in-app "Keep local"/"Use server" resolution.
  Per-item cloud_done/sync badges show on Files tiles.
- Share/Download short-circuit to the local copy for already-synced
  files instead of a fresh network fetch.
- Settings gained a Device Sync section (per-folder list, "Sync
  everything", Wi-Fi-only toggle, manual "Sync now").
- Fixed two release-only bugs found via on-device testing: Android's
  HttpURLConnection silently rejects the PROPFIND method (switched to
  OkHttp), and R8 was stripping WorkManager's reflection-instantiated
  internals (broadened proguard-rules.pro to keep androidx.work.**
  wholesale rather than chasing individual classes).

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

lib/widgets/share_sheet.dart

@@ -236,9 +236,17 @@ class _ShareSheetState extends State<ShareSheet> {
     setState(() => _isSharingFile = true);
     final messenger = ScaffoldMessenger.of(context);
     try {
-      final tempDir = await getTemporaryDirectory();
-      final tempPath = p.join(tempDir.path, widget.item.name);
-      await provider.service!.downloadToFile(widget.item.path, tempPath);
+      // Already mirrored locally by device sync? Share that copy straight
+      // away instead of a fresh WebDAV fetch - see
+      // ServerProvider.localSyncedFilePath.
+      final tempPath =
+          await provider.localSyncedFilePath(widget.item) ??
+          await () async {
+            final tempDir = await getTemporaryDirectory();
+            final path = p.join(tempDir.path, widget.item.name);
+            await provider.service!.downloadToFile(widget.item.path, path);
+            return path;
+          }();
       if (!mounted) return;
       await SharePlus.instance.share(
         ShareParams(