feat(boxing): handle multi-code paichan switches

lib/services/feedback_service.dart

@@ -11,6 +11,7 @@ enum FeedbackEvent {
   networkError, // Network error         → failure sound + short vibrate
   duplicateBoxNo, // Duplicate box no     → failure sound + short vibrate
   modeSwitch, // Mode toggle           → beep only
+  paichanSwitch, // Paichan changed      → beep + short vibrate
 }
 
 /// Centralized feedback dispatcher.
@@ -56,6 +57,10 @@ class FeedbackService {
 
       case FeedbackEvent.modeSwitch:
         _soundService.playBeep();
+
+      case FeedbackEvent.paichanSwitch:
+        _soundService.playBeep();
+        _vibrateShort();
     }
   }