improve single-code boxing UX: preserve state, fix work order display, add double-vibrate alert

- Keep UI state after boxing completion for operator review instead of auto-reset
- Show real work order number instead of hardcoded "本次扫码" in assigned list
- Add double-vibration feedback when re-scanning already completed zongpai
- Change completed status banner and dot from green to red for clearer warning

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/services/feedback_service.dart

@@ -10,6 +10,7 @@ enum FeedbackEvent {
   duplicateError, // Duplicate (shelf)    → alert loop + continuous vibrate
   networkError, // Network error         → failure sound + short vibrate
   duplicateBoxNo, // Duplicate box no     → failure sound + short vibrate
+  alreadyCompleted, // Zongpai already boxed → beep + double vibrate
   modeSwitch, // Mode toggle           → beep only
   paichanSwitch, // Paichan changed      → beep + short vibrate
 }
@@ -55,6 +56,10 @@ class FeedbackService {
         _soundService.playFailure();
         _vibrateShort();
 
+      case FeedbackEvent.alreadyCompleted:
+        _soundService.playBeep();
+        _vibrateDouble();
+
       case FeedbackEvent.modeSwitch:
         _soundService.playBeep();
 
@@ -82,6 +87,11 @@ class FeedbackService {
     Vibration.vibrate(pattern: [0, 300, 100, 300, 100, 300]);
   }
 
+  /// Two short bursts to signal an already-completed item.
+  void _vibrateDouble() {
+    Vibration.vibrate(pattern: [0, 200, 100, 200]);
+  }
+
   Future<void> dispose() async {
     await _soundService.dispose();
   }