feat(ui): 화면 및 공통 위젯 개선

- FrontScreen 개선
- GamePlayScreen, GameSessionController 업데이트
- ArenaBattleScreen, NewCharacterScreen 정리
- AsciiDisintegrateWidget 추가

lib/src/features/new_character/new_character_screen.dart

@@ -259,7 +259,7 @@ class _NewCharacterScreenState extends State<NewCharacterScreen> {
       seed: gameSeed,
       traits: traits,
       stats: finalStats,
-      inventory: const Inventory(gold: 0, items: []),
+      inventory: Inventory.empty(),
       equipment: Equipment.empty(),
       skillBook: SkillBook.empty(),
       progress: ProgressState.empty(),