refactor(auth): Migrate session storage to flutter_secure_storage

lib/features/preferences/data/repositories/local_preferences_repository.dart

@@ -1,5 +1,6 @@
 import 'dart:convert';
 
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 import 'package:injectable/injectable.dart';
 import 'package:moonwell_launcher/features/launcher/domain/entities/launcher_session.dart';
 import 'package:moonwell_launcher/features/preferences/domain/repositories/preferences_repository.dart';
@@ -9,12 +10,15 @@ const _outputDirKey = 'output_dir';
 const _launcherSessionKey = 'launcher_session';
 
 @LazySingleton(as: PreferencesRepository, env: ['flutter'])
-class SharedPrefsPreferencesRepository implements PreferencesRepository {
+class LocalPreferencesRepository implements PreferencesRepository {
   final SharedPreferences _preferences;
+  final FlutterSecureStorage _secureStorage;
 
-  const SharedPrefsPreferencesRepository({
+  const LocalPreferencesRepository({
     required SharedPreferences preferences,
-  }) : _preferences = preferences;
+    required FlutterSecureStorage secureStorage,
+  }) : _preferences = preferences,
+       _secureStorage = secureStorage;
 
   @override
   Future<Uri?> getOutputDir() {
@@ -33,35 +37,45 @@ class SharedPrefsPreferencesRepository implements PreferencesRepository {
   }
 
   @override
-  Future<LauncherSession?> getLauncherSession() {
-    final rawSession = _preferences.getString(_launcherSessionKey);
+  Future<LauncherSession?> getLauncherSession() async {
+    await _maybeClearLegacyLauncherSession();
+
+    final rawSession = await _secureStorage.read(key: _launcherSessionKey);
     if (rawSession == null || rawSession.isEmpty) {
-      return Future.value(null);
+      return null;
     }
 
     try {
       final decoded = jsonDecode(rawSession);
       if (decoded is! Map) {
-        return Future.value(null);
+        return null;
       }
 
       final json = decoded.map((key, value) => MapEntry(key.toString(), value));
-      return Future.value(LauncherSession.fromJson(json));
+      return LauncherSession.fromJson(json);
     } catch (_) {
-      return Future.value(null);
+      return null;
     }
   }
 
   @override
-  Future<void> setLauncherSession(LauncherSession session) {
-    return _preferences.setString(
-      _launcherSessionKey,
-      jsonEncode(session.toJson()),
+  Future<void> setLauncherSession(LauncherSession session) async {
+    await _secureStorage.write(
+      key: _launcherSessionKey,
+      value: jsonEncode(session.toJson()),
     );
+    await _maybeClearLegacyLauncherSession();
   }
 
   @override
-  Future<void> clearLauncherSession() {
-    return _preferences.remove(_launcherSessionKey);
+  Future<void> clearLauncherSession() async {
+    await _secureStorage.delete(key: _launcherSessionKey);
+    await _maybeClearLegacyLauncherSession();
+  }
+
+  Future<void> _maybeClearLegacyLauncherSession() async {
+    if (_preferences.containsKey(_launcherSessionKey)) {
+      await _preferences.remove(_launcherSessionKey);
+    }
   }
 }