feat(ui): 도움말 다이얼로그 및 UI 개선

- HelpDialog 추가
- 게임 화면에 통계/도움말 버튼 추가
- CombatLog에 디버프 이벤트 표시
- AudioService mp3 확장자 지원
- 설정 텍스트 l10n 추가

lib/src/app.dart

@@ -161,10 +161,12 @@ class _AskiiNeverDieAppState extends State<AskiiNeverDieApp> {
         },
         currentThemeMode: _themeMode,
         onThemeModeChange: _changeThemeMode,
+        audioService: _audioService,
       );
     }
 
-    // 세이브 파일이 없으면 기존 프론트 화면
+    // 세이브 파일이 없으면 기존 프론트 화면 (타이틀 BGM 재생)
+    _audioService.playBgm('title');
     return FrontScreen(
       onNewCharacter: _navigateToNewCharacter,
       onLoadSave: _loadSave,
@@ -238,6 +240,7 @@ class _AskiiNeverDieAppState extends State<AskiiNeverDieApp> {
         MaterialPageRoute<void>(
           builder: (context) => GamePlayScreen(
             controller: _controller,
+            audioService: _audioService,
             forceCarouselLayout: testMode,
             currentThemeMode: _themeMode,
             onThemeModeChange: _changeThemeMode,
@@ -252,6 +255,7 @@ class _AskiiNeverDieAppState extends State<AskiiNeverDieApp> {
       MaterialPageRoute<void>(
         builder: (context) => GamePlayScreen(
           controller: _controller,
+          audioService: _audioService,
           currentThemeMode: _themeMode,
           onThemeModeChange: _changeThemeMode,
         ),
@@ -298,12 +302,14 @@ class _AutoLoadScreen extends StatefulWidget {
     required this.onLoadFailed,
     required this.currentThemeMode,
     required this.onThemeModeChange,
+    this.audioService,
   });
 
   final GameSessionController controller;
   final VoidCallback onLoadFailed;
   final ThemeMode currentThemeMode;
   final void Function(ThemeMode mode) onThemeModeChange;
+  final AudioService? audioService;
 
   @override
   State<_AutoLoadScreen> createState() => _AutoLoadScreenState();
@@ -313,6 +319,8 @@ class _AutoLoadScreenState extends State<_AutoLoadScreen> {
   @override
   void initState() {
     super.initState();
+    // 로딩 중에도 타이틀 BGM 재생
+    widget.audioService?.playBgm('title');
     _autoLoad();
   }
 
@@ -327,6 +335,7 @@ class _AutoLoadScreenState extends State<_AutoLoadScreen> {
         MaterialPageRoute<void>(
           builder: (context) => GamePlayScreen(
             controller: widget.controller,
+            audioService: widget.audioService,
             currentThemeMode: widget.currentThemeMode,
             onThemeModeChange: widget.onThemeModeChange,
           ),