update

lib/shared/services/storage_service.dart

@@ -0,0 +1,36 @@
+import 'dart:convert';
+import 'package:shared_preferences/shared_preferences.dart';
+import '../models/transaction.dart';
+
+class StorageService {
+  static const _key = 'transactions';
+
+  final SharedPreferences _prefs;
+
+  StorageService(this._prefs);
+
+  List<Transaction> loadTransactions() {
+    final raw = _prefs.getString(_key);
+    if (raw == null) return [];
+    final list = jsonDecode(raw) as List<dynamic>;
+    return list
+        .map((e) => Transaction.fromJson(e as Map<String, dynamic>))
+        .toList();
+  }
+
+  Future<void> saveTransactions(List<Transaction> transactions) async {
+    final encoded = jsonEncode(transactions.map((t) => t.toJson()).toList());
+    await _prefs.setString(_key, encoded);
+  }
+
+  Future<void> addTransaction(Transaction transaction) async {
+    final list = loadTransactions();
+    list.add(transaction);
+    await saveTransactions(list);
+  }
+
+  Future<void> deleteTransaction(String id) async {
+    final list = loadTransactions()..removeWhere((t) => t.id == id);
+    await saveTransactions(list);
+  }
+}