add custom srs

lib/src/services/custom_deck_repository.dart

@@ -0,0 +1,30 @@
+
+import 'dart:convert';
+import 'package:shared_preferences/shared_preferences.dart';
+import '../models/custom_kanji_item.dart';
+
+class CustomDeckRepository {
+  static const _key = 'custom_deck';
+
+  Future<List<CustomKanjiItem>> getCustomDeck() async {
+    final prefs = await SharedPreferences.getInstance();
+    final jsonString = prefs.getString(_key);
+    if (jsonString != null) {
+      final List<dynamic> jsonList = json.decode(jsonString);
+      return jsonList.map((json) => CustomKanjiItem.fromJson(json)).toList();
+    }
+    return [];
+  }
+
+  Future<void> addCard(CustomKanjiItem item) async {
+    final deck = await getCustomDeck();
+    deck.add(item);
+    await _saveDeck(deck);
+  }
+
+  Future<void> _saveDeck(List<CustomKanjiItem> deck) async {
+    final prefs = await SharedPreferences.getInstance();
+    final jsonList = deck.map((item) => item.toJson()).toList();
+    await prefs.setString(_key, json.encode(jsonList));
+  }
+}