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();
+}

lib/features/sync/storage/sync_state_store_hive.dart

@@ -0,0 +1,46 @@
+import 'package:hive_flutter/hive_flutter.dart';
+import 'package:relationship_saver/features/sync/storage/sync_state_store.dart';
+
+/// Hive-backed sync state store.
+class HiveSyncStateStore implements SyncStateStore {
+  static const String _boxName = 'relationship_saver_sync';
+  static const String _stateKey = 'sync_state_json';
+
+  static bool _initialized = false;
+
+  @override
+  Future<void> clear() async {
+    final Box<dynamic> box = await _openBox();
+    await box.delete(_stateKey);
+  }
+
+  @override
+  Future<SyncStateRecord?> read() async {
+    final Box<dynamic> box = await _openBox();
+    final String? rawState = box.get(_stateKey) as String?;
+    if (rawState == null || rawState.isEmpty) {
+      return null;
+    }
+
+    return SyncStateRecord(rawState: rawState);
+  }
+
+  @override
+  Future<void> write({required String rawState}) async {
+    final Box<dynamic> box = await _openBox();
+    await box.put(_stateKey, rawState);
+  }
+
+  Future<Box<dynamic>> _openBox() async {
+    if (!_initialized) {
+      await Hive.initFlutter();
+      _initialized = true;
+    }
+
+    if (!Hive.isBoxOpen(_boxName)) {
+      return Hive.openBox<dynamic>(_boxName);
+    }
+
+    return Hive.box<dynamic>(_boxName);
+  }
+}

lib/features/sync/storage/sync_state_store_in_memory.dart

@@ -0,0 +1,19 @@
+import 'package:relationship_saver/features/sync/storage/sync_state_store.dart';
+
+/// In-memory sync state store for tests.
+class InMemorySyncStateStore implements SyncStateStore {
+  SyncStateRecord? _record;
+
+  @override
+  Future<void> clear() async {
+    _record = null;
+  }
+
+  @override
+  Future<SyncStateRecord?> read() async => _record;
+
+  @override
+  Future<void> write({required String rawState}) async {
+    _record = SyncStateRecord(rawState: rawState);
+  }
+}

lib/features/sync/storage/sync_state_store_provider.dart

@@ -0,0 +1,14 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:relationship_saver/core/config/app_config.dart';
+import 'package:relationship_saver/features/sync/storage/sync_state_store.dart';
+import 'package:relationship_saver/features/sync/storage/sync_state_store_hive.dart';
+import 'package:relationship_saver/features/sync/storage/sync_state_store_shared_prefs.dart';
+
+/// Chooses sync queue persistence backend.
+final Provider<SyncStateStore> syncStateStoreProvider =
+    Provider<SyncStateStore>((Ref ref) {
+      if (AppConfig.useHiveLocalDb) {
+        return HiveSyncStateStore();
+      }
+      return SharedPrefsSyncStateStore();
+    });

lib/features/sync/storage/sync_state_store_shared_prefs.dart

@@ -0,0 +1,32 @@
+import 'package:relationship_saver/features/sync/storage/sync_state_store.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+/// shared_preferences-backed sync state store.
+class SharedPrefsSyncStateStore implements SyncStateStore {
+  SharedPrefsSyncStateStore({this.stateKey = 'sync_queue_state_v1'});
+
+  final String stateKey;
+
+  @override
+  Future<void> clear() async {
+    final SharedPreferences prefs = await SharedPreferences.getInstance();
+    await prefs.remove(stateKey);
+  }
+
+  @override
+  Future<SyncStateRecord?> read() async {
+    final SharedPreferences prefs = await SharedPreferences.getInstance();
+    final String? rawState = prefs.getString(stateKey);
+    if (rawState == null || rawState.isEmpty) {
+      return null;
+    }
+
+    return SyncStateRecord(rawState: rawState);
+  }
+
+  @override
+  Future<void> write({required String rawState}) async {
+    final SharedPreferences prefs = await SharedPreferences.getInstance();
+    await prefs.setString(stateKey, rawState);
+  }
+}

lib/features/sync/sync_queue_repository.dart

@@ -1,27 +1,43 @@
 import 'dart:convert';
 
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:relationship_saver/core/config/app_config.dart';
+import 'package:relationship_saver/features/sync/storage/sync_state_store.dart';
+import 'package:relationship_saver/features/sync/storage/sync_state_store_hive.dart';
+import 'package:relationship_saver/features/sync/storage/sync_state_store_provider.dart';
+import 'package:relationship_saver/features/sync/storage/sync_state_store_shared_prefs.dart';
 import 'package:relationship_saver/features/sync/sync_state.dart';
 import 'package:relationship_saver/integrations/backend/models/backend_models.dart';
-import 'package:shared_preferences/shared_preferences.dart';
 
 /// Persists queued local mutations and sync cursor metadata.
 class SyncQueueRepository extends AsyncNotifier<SyncState> {
-  static const String _storageKey = 'sync_queue_state_v1';
-
   @override
   Future<SyncState> build() async {
-    final SharedPreferences prefs = await SharedPreferences.getInstance();
-    final String? raw = prefs.getString(_storageKey);
-    if (raw == null || raw.isEmpty) {
+    final SyncStateStore store = ref.read(syncStateStoreProvider);
+    SyncStateRecord? record = await store.read();
+
+    if ((record == null || record.rawState.isEmpty) &&
+        AppConfig.useHiveLocalDb &&
+        store is HiveSyncStateStore) {
+      final SharedPrefsSyncStateStore legacyStore = SharedPrefsSyncStateStore();
+      final SyncStateRecord? legacy = await legacyStore.read();
+      if (legacy != null && legacy.rawState.isNotEmpty) {
+        await store.write(rawState: legacy.rawState);
+        await legacyStore.clear();
+        record = await store.read();
+      }
+    }
+
+    if (record == null || record.rawState.isEmpty) {
       return SyncState.empty;
     }
 
     try {
-      final Map<String, dynamic> json = jsonDecode(raw) as Map<String, dynamic>;
+      final Map<String, dynamic> json =
+          jsonDecode(record.rawState) as Map<String, dynamic>;
       return SyncState.fromJson(json);
     } on FormatException {
-      await prefs.remove(_storageKey);
+      await store.clear();
       return SyncState.empty;
     }
   }
@@ -121,8 +137,9 @@ class SyncQueueRepository extends AsyncNotifier<SyncState> {
 
   Future<void> _setState(SyncState next) async {
     state = AsyncData<SyncState>(next);
-    final SharedPreferences prefs = await SharedPreferences.getInstance();
-    await prefs.setString(_storageKey, jsonEncode(next.toJson()));
+    await ref
+        .read(syncStateStoreProvider)
+        .write(rawState: jsonEncode(next.toJson()));
   }
 }