feat(audio): 오디오 시스템 추가

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

macos/Flutter/GeneratedPluginRegistrant.swift

@@ -5,10 +5,14 @@
 import FlutterMacOS
 import Foundation
 
+import audio_session
+import just_audio
 import path_provider_foundation
 import shared_preferences_foundation
 
 func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
+  AudioSessionPlugin.register(with: registry.registrar(forPlugin: "AudioSessionPlugin"))
+  JustAudioPlugin.register(with: registry.registrar(forPlugin: "JustAudioPlugin"))
   PathProviderPlugin.register(with: registry.registrar(forPlugin: "PathProviderPlugin"))
   SharedPreferencesPlugin.register(with: registry.registrar(forPlugin: "SharedPreferencesPlugin"))
 }