refactor(model): freezed 패키지 도입으로 보일러플레이트 제거

- ItemStats, CombatStats, EquipmentItem을 freezed로 마이그레이션
- copyWith, toJson/fromJson 자동 생성
- 세이브 파일 호환성 유지

pubspec.yaml

@@ -38,11 +38,18 @@ dependencies:
   path_provider: ^2.1.4
   shared_preferences: ^2.3.1
   just_audio: ^0.9.42
+  # Code generation annotations
+  freezed_annotation: ^2.4.1
+  json_annotation: ^4.9.0
 
 dev_dependencies:
   flutter_test:
     sdk: flutter
   fake_async: ^1.3.2
+  # Code generation
+  build_runner: ^2.4.13
+  freezed: ^2.5.7
+  json_serializable: ^6.8.0
 
   # The "flutter_lints" package below contains a set of recommended lints to
   # encourage good coding practices. The lint set provided by the package is