fix(notification): Act 완료 토스트 메시지 인덱싱 버그 수정

- plotStageCount 인덱싱 오류 수정 (plotStageCount - 1 → -2)
- 프롤로그 완료 시 "PROLOGUE COMPLETE!" 표시되도록 수정
- showActComplete(0)이 프롤로그 완료를 올바르게 처리

lib/src/core/notification/notification_service.dart

@@ -77,10 +77,14 @@ class NotificationService {
   }
 
   /// 막 완료 알림 (Act Complete)
+  /// actNumber: 0=프롤로그, 1=Act I, 2=Act II, ...
   void showActComplete(int actNumber) {
+    final title = actNumber == 0
+        ? 'PROLOGUE COMPLETE!'
+        : 'ACT $actNumber COMPLETE!';
     show(GameNotification(
       type: NotificationType.actComplete,
-      title: 'ACT $actNumber COMPLETE!',
+      title: title,
       duration: const Duration(seconds: 3),
     ));
   }

lib/src/features/game/game_play_screen.dart

@@ -105,10 +105,12 @@ class _GamePlayScreenState extends State<GamePlayScreen>
     _lastQuestCount = state.progress.questCount;
 
     // Act 완료 감지 (plotStageCount 증가)
+    // plotStageCount: 1=프롤로그 진행, 2=프롤로그 완료, 3=Act1 완료...
+    // 완료된 스테이지 인덱스 = plotStageCount - 2 (0=프롤로그, 1=Act1, ...)
     if (state.progress.plotStageCount > _lastPlotStageCount &&
         _lastPlotStageCount > 0) {
       _specialAnimation = AsciiAnimationType.actComplete;
-      _notificationService.showActComplete(state.progress.plotStageCount - 1);
+      _notificationService.showActComplete(state.progress.plotStageCount - 2);
       _resetSpecialAnimationAfterFrame();
     }
     _lastPlotStageCount = state.progress.plotStageCount;