Migrate sync queue persistence to store adapters

test/features/sync/sync_queue_repository_test.dart

@@ -1,16 +1,16 @@
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:relationship_saver/features/sync/storage/sync_state_store_in_memory.dart';
+import 'package:relationship_saver/features/sync/storage/sync_state_store_provider.dart';
 import 'package:relationship_saver/features/sync/sync_queue_repository.dart';
 import 'package:relationship_saver/integrations/backend/models/backend_models.dart';
-import 'package:shared_preferences/shared_preferences.dart';
 
 void main() {
-  setUp(() {
-    SharedPreferences.setMockInitialValues(<String, Object>{});
-  });
-
   test('enqueue persists and can be reloaded', () async {
-    final ProviderContainer container = ProviderContainer();
+    final InMemorySyncStateStore store = InMemorySyncStateStore();
+    final ProviderContainer container = ProviderContainer(
+      overrides: [syncStateStoreProvider.overrideWithValue(store)],
+    );
 
     await container
         .read(syncQueueRepositoryProvider.notifier)
@@ -20,7 +20,9 @@ void main() {
 
     container.dispose();
 
-    final ProviderContainer reloaded = ProviderContainer();
+    final ProviderContainer reloaded = ProviderContainer(
+      overrides: [syncStateStoreProvider.overrideWithValue(store)],
+    );
     addTearDown(reloaded.dispose);
     final SyncStateData reloadedState = await _readState(reloaded);
     expect(reloadedState.pending.length, 1);
@@ -30,7 +32,11 @@ void main() {
   test(
     'applyPushResult removes completed mutation ids and tracks rejections',
     () async {
-      final ProviderContainer container = ProviderContainer();
+      final ProviderContainer container = ProviderContainer(
+        overrides: [
+          syncStateStoreProvider.overrideWithValue(InMemorySyncStateStore()),
+        ],
+      );
       addTearDown(container.dispose);
 
       final SyncQueueRepository notifier = container.read(