Add local data store adapter layer and Hive scaffold

lib/features/local/storage/local_data_store.dart

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+/// Raw local state payload and associated schema version.
+class LocalDataRecord {
+  const LocalDataRecord({required this.rawState, required this.schemaVersion});
+
+  final String rawState;
+  final int schemaVersion;
+}
+
+/// Persistence boundary for local app state.
+abstract interface class LocalDataStore {
+  /// Reads latest persisted state payload.
+  Future<LocalDataRecord?> read();
+
+  /// Writes state payload and schema version.
+  Future<void> write({required String rawState, required int schemaVersion});
+
+  /// Clears persisted state payload.
+  Future<void> clear();
+}

lib/features/local/storage/local_data_store_hive.dart

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+import 'package:hive_flutter/hive_flutter.dart';
+import 'package:relationship_saver/features/local/storage/local_data_store.dart';
+
+/// Hive-backed local store for future DB migration path.
+class HiveLocalDataStore implements LocalDataStore {
+  static const String _boxName = 'relationship_saver_local';
+  static const String _stateKey = 'state_json';
+  static const String _schemaVersionKey = 'schema_version';
+
+  static bool _initialized = false;
+
+  @override
+  Future<void> clear() async {
+    final Box<dynamic> box = await _openBox();
+    await box.delete(_stateKey);
+    await box.delete(_schemaVersionKey);
+  }
+
+  @override
+  Future<LocalDataRecord?> read() async {
+    final Box<dynamic> box = await _openBox();
+    final String? rawState = box.get(_stateKey) as String?;
+    if (rawState == null || rawState.isEmpty) {
+      return null;
+    }
+
+    final int schemaVersion =
+        (box.get(_schemaVersionKey) as num?)?.toInt() ?? 0;
+    return LocalDataRecord(rawState: rawState, schemaVersion: schemaVersion);
+  }
+
+  @override
+  Future<void> write({
+    required String rawState,
+    required int schemaVersion,
+  }) async {
+    final Box<dynamic> box = await _openBox();
+    await box.put(_stateKey, rawState);
+    await box.put(_schemaVersionKey, schemaVersion);
+  }
+
+  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/local/storage/local_data_store_in_memory.dart

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+import 'package:relationship_saver/features/local/storage/local_data_store.dart';
+
+/// In-memory local store for deterministic tests.
+class InMemoryLocalDataStore implements LocalDataStore {
+  LocalDataRecord? _record;
+
+  @override
+  Future<void> clear() async {
+    _record = null;
+  }
+
+  @override
+  Future<LocalDataRecord?> read() async => _record;
+
+  @override
+  Future<void> write({
+    required String rawState,
+    required int schemaVersion,
+  }) async {
+    _record = LocalDataRecord(rawState: rawState, schemaVersion: schemaVersion);
+  }
+}

lib/features/local/storage/local_data_store_provider.dart

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+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:relationship_saver/core/config/app_config.dart';
+import 'package:relationship_saver/features/local/storage/local_data_store.dart';
+import 'package:relationship_saver/features/local/storage/local_data_store_hive.dart';
+import 'package:relationship_saver/features/local/storage/local_data_store_shared_prefs.dart';
+
+/// Chooses local persistence backend for app state.
+final Provider<LocalDataStore> localDataStoreProvider =
+    Provider<LocalDataStore>((Ref ref) {
+      if (AppConfig.useHiveLocalDb) {
+        return HiveLocalDataStore();
+      }
+      return SharedPrefsLocalDataStore();
+    });

lib/features/local/storage/local_data_store_shared_prefs.dart

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+import 'package:relationship_saver/features/local/storage/local_data_store.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+/// shared_preferences-backed local store used as stable default.
+class SharedPrefsLocalDataStore implements LocalDataStore {
+  SharedPrefsLocalDataStore({
+    this.stateKey = 'local_data_state_v1',
+    this.schemaVersionKey = 'local_data_schema_version',
+  });
+
+  final String stateKey;
+  final String schemaVersionKey;
+
+  @override
+  Future<void> clear() async {
+    final SharedPreferences prefs = await SharedPreferences.getInstance();
+    await prefs.remove(stateKey);
+    await prefs.remove(schemaVersionKey);
+  }
+
+  @override
+  Future<LocalDataRecord?> read() async {
+    final SharedPreferences prefs = await SharedPreferences.getInstance();
+    final String? rawState = prefs.getString(stateKey);
+    if (rawState == null || rawState.isEmpty) {
+      return null;
+    }
+
+    return LocalDataRecord(
+      rawState: rawState,
+      schemaVersion: prefs.getInt(schemaVersionKey) ?? 0,
+    );
+  }
+
+  @override
+  Future<void> write({
+    required String rawState,
+    required int schemaVersion,
+  }) async {
+    final SharedPreferences prefs = await SharedPreferences.getInstance();
+    await prefs.setString(stateKey, rawState);
+    await prefs.setInt(schemaVersionKey, schemaVersion);
+  }
+}