feat(audio): 오디오 시스템 추가

- just_audio 패키지 추가
- AudioService 구현 (BGM/SFX 재생)
- assets/audio/bgm/, assets/audio/sfx/ 에셋 추가

pubspec.yaml

@@ -37,6 +37,7 @@ dependencies:
   intl: ^0.20.2
   path_provider: ^2.1.4
   shared_preferences: ^2.3.1
+  just_audio: ^0.9.42
 
 dev_dependencies:
   flutter_test:
@@ -65,6 +66,8 @@ flutter:
   # To add assets to your application, add an assets section, like this:
   assets:
     - assets/
+    - assets/audio/bgm/
+    - assets/audio/sfx/
 
   # An image asset can refer to one or more resolution-specific "variants", see
   # https://flutter.dev/to/resolution-aware-images