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/services/account_store.dart

@@ -37,6 +37,8 @@ class AccountStore {
     'ui_files_type_filter',
     'ui_cache_policy',
     'ui_cache_interval_minutes',
+    'ui_synced_folders',
+    'ui_sync_everything',
   ];
 
   String accountPrefKey(String accountId, String baseKey) =>

lib/services/sync_service.dart

@@ -0,0 +1,144 @@
+import 'dart:convert';
+import 'package:flutter/services.dart';
+import '../models/sync_status.dart';
+import '../providers/server_provider.dart';
+
+/// A snapshot of native device-sync status - see `SyncStatusBus.kt` (the
+/// in-process pub/sub it mirrors) and `MainActivity.kt`'s `syncStatusMap`.
+class SyncStatusSnapshot {
+  final String? accountId;
+  final bool syncing;
+  final Set<String> syncingFileIds;
+  final Set<String> syncedFileIds;
+  final List<SyncConflictInfo> conflicts;
+
+  const SyncStatusSnapshot({
+    required this.accountId,
+    required this.syncing,
+    required this.syncingFileIds,
+    required this.syncedFileIds,
+    required this.conflicts,
+  });
+
+  factory SyncStatusSnapshot.fromMap(Map<dynamic, dynamic> map) {
+    return SyncStatusSnapshot(
+      accountId: map['accountId'] as String?,
+      syncing: map['syncing'] as bool? ?? false,
+      syncingFileIds: ((map['syncingFileIds'] as List?) ?? const [])
+          .cast<String>()
+          .toSet(),
+      syncedFileIds: ((map['syncedFileIds'] as List?) ?? const [])
+          .cast<String>()
+          .toSet(),
+      conflicts: ((map['conflicts'] as List?) ?? const [])
+          .cast<Map<dynamic, dynamic>>()
+          .map(SyncConflictInfo.fromMap)
+          .toList(),
+    );
+  }
+}
+
+/// Talks to `SyncWorker`/`SyncEngine.kt`'s Android WorkManager-based device
+/// sync (see `server.md`'s "Device sync" section for the full design) -
+/// Dart's job here is only to gather what the native side needs, push a
+/// fresh config whenever it changes, and listen for live status; the actual
+/// PROPFIND-walk/diff/GET/PUT engine runs entirely in Kotlin, independent
+/// of the Flutter engine.
+class SyncService {
+  static const _channel = MethodChannel('dev.ayushya.noo/sync_service');
+  static const _statusChannel = EventChannel(
+    'dev.ayushya.noo/sync_service/status',
+  );
+
+  /// Cancels (if [ServerProvider.syncedPaths] is now empty) or re-enqueues
+  /// the periodic sync job with fresh account/credentials/path-list/
+  /// network-constraint data - call this any time one of those changes,
+  /// since a periodic `WorkRequest`'s input is fixed at enqueue time and
+  /// can only be updated by re-enqueueing.
+  static Future<void> reschedule(ServerProvider provider) async {
+    final accountId = provider.activeAccountId;
+    final service = provider.service;
+    final authHeader = service?.authHeaders['Authorization'];
+    if (accountId == null || service == null || authHeader == null) {
+      return cancel();
+    }
+
+    final paths = provider.syncEverything ? ['/'] : provider.syncedPaths;
+    await _channel.invokeMethod('reschedule', {
+      'accountId': accountId,
+      'serverUrl': provider.serverUrl,
+      'username': provider.username,
+      'authHeader': authHeader,
+      'folders': jsonEncode(paths),
+      'wifiOnly': !provider.syncOnCellular,
+    });
+  }
+
+  static Future<void> cancel() async {
+    await _channel.invokeMethod('cancel');
+  }
+
+  /// Runs a one-off sync pass immediately (Settings' "Sync now"),
+  /// independent of the periodic schedule.
+  static Future<void> syncNow(ServerProvider provider) async {
+    final accountId = provider.activeAccountId;
+    final service = provider.service;
+    final authHeader = service?.authHeaders['Authorization'];
+    if (accountId == null || service == null || authHeader == null) {
+      throw Exception('Not logged in.');
+    }
+
+    final paths = provider.syncEverything ? ['/'] : provider.syncedPaths;
+    await _channel.invokeMethod('syncNow', {
+      'accountId': accountId,
+      'serverUrl': provider.serverUrl,
+      'username': provider.username,
+      'authHeader': authHeader,
+      'folders': jsonEncode(paths),
+    });
+  }
+
+  /// One-shot status snapshot - used to seed [ServerProvider]'s state right
+  /// after login/account-switch, before the first [statusStream] event
+  /// arrives.
+  static Future<SyncStatusSnapshot> getStatus() async {
+    final result = await _channel.invokeMethod<Map<dynamic, dynamic>>(
+      'getSyncStatus',
+    );
+    return SyncStatusSnapshot.fromMap(result ?? const {});
+  }
+
+  /// Live status updates pushed from `SyncStatusBus` (Kotlin) - syncing
+  /// started/stopped, per-file progress, new/resolved conflicts.
+  static Stream<SyncStatusSnapshot> get statusStream {
+    return _statusChannel.receiveBroadcastStream().map(
+      (event) => SyncStatusSnapshot.fromMap(event as Map<dynamic, dynamic>),
+    );
+  }
+
+  /// In-app conflict resolution (the header's "Keep local"/"Use server"
+  /// buttons) - shares the exact same native `ConflictResolveWorker`
+  /// enqueue path the notification actions use, just triggered from Dart
+  /// instead of a `PendingIntent`.
+  static Future<void> resolveConflict(
+    ServerProvider provider,
+    SyncConflictInfo conflict,
+    String resolution,
+  ) async {
+    final service = provider.service;
+    final authHeader = service?.authHeaders['Authorization'];
+    if (service == null || authHeader == null) {
+      throw Exception('Not logged in.');
+    }
+    await _channel.invokeMethod('resolveConflict', {
+      'accountId': conflict.accountId,
+      'serverUrl': provider.serverUrl,
+      'username': provider.username,
+      'authHeader': authHeader,
+      'fileId': conflict.fileId,
+      'remotePath': conflict.remotePath,
+      'relPath': conflict.relPath,
+      'resolution': resolution,
+    });
+  }
+}