feat: add local-first private AI digest workflow

Migrate app code into canonical feature slices, add phone-only AI digest scheduling and review, wire local notification/background task support, and cover the flow with tests.

lib/features/sync/storage/sync_state_store.dart

@@ -1,18 +1,2 @@
-/// Raw persisted payload for sync queue state.
-class SyncStateRecord {
-  const SyncStateRecord({required this.rawState});
-
-  final String rawState;
-}
-
-/// Persistence boundary for sync queue metadata and pending changes.
-abstract interface class SyncStateStore {
-  /// Reads stored sync state payload.
-  Future<SyncStateRecord?> read();
-
-  /// Writes sync state payload.
-  Future<void> write({required String rawState});
-
-  /// Clears stored sync state payload.
-  Future<void> clear();
-}
+// Legacy compatibility export for the sync state store contract.
+export 'package:relationship_saver/features/sync/data/storage/sync_state_store.dart';