Migrate sync queue persistence to store adapters

lib/features/sync/storage/sync_state_store.dart

@@ -0,0 +1,18 @@
+/// 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();
+}