refactor: unify error feedback system with FeedbackService and StatusBar widget

- Create FeedbackService as centralized sound/vibration dispatcher
- Create reusable StatusBar widget with StatusDotColor enum (blue/orange/green/red/yellow/amber)
- Extend SoundService with beep/error/alert sound types and loop playback
- Remove top feedback banners from both registration and boxing pages
- Route all feedback through bottom status bar per PRD requirements
- Add sound and vibration feedback to boxing module (was completely missing)
- Handle network errors with yellow status, general errors with red
- Enhance duplicate dialog with alert loop sound that stops on close
- Add beep/error/alert sound file selectors in settings page

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

lib/services/feedback_service.dart

@@ -0,0 +1,83 @@
+import 'package:vibration/vibration.dart';
+import 'package:pad_scanner/services/sound_service.dart';
+
+/// Unified feedback events used across the app.
+enum FeedbackEvent {
+  scanValid, // Valid barcode scanned  → beep + short vibrate
+  scanInvalid, // Invalid barcode       → error sound + short vibrate
+  submitSuccess, // Submit succeeded     → success sound + long vibrate
+  submitFailure, // Submit failed        → failure sound + short vibrate
+  duplicateError, // Duplicate (shelf)    → alert loop + continuous vibrate
+  networkError, // Network error         → failure sound + short vibrate
+  duplicateBoxNo, // Duplicate box no     → failure sound + short vibrate
+  modeSwitch, // Mode toggle           → beep only
+}
+
+/// Centralized feedback dispatcher.
+///
+/// Each page manages its own bottom status bar state; this service handles
+/// the sound, vibration channels only.
+class FeedbackService {
+  final SoundService _soundService;
+
+  FeedbackService({SoundService? soundService})
+    : _soundService = soundService ?? SoundService();
+
+  /// Trigger feedback for the given [event].
+  Future<void> trigger(FeedbackEvent event) async {
+    switch (event) {
+      case FeedbackEvent.scanValid:
+        _soundService.playBeep();
+        _vibrateShort();
+
+      case FeedbackEvent.scanInvalid:
+        _soundService.playError();
+        _vibrateShort();
+
+      case FeedbackEvent.submitSuccess:
+        _soundService.playSuccess();
+        _vibrateLong();
+
+      case FeedbackEvent.submitFailure:
+        _soundService.playFailure();
+        _vibrateShort();
+
+      case FeedbackEvent.duplicateError:
+        _soundService.playAlertLoop();
+        _vibratePattern();
+
+      case FeedbackEvent.networkError:
+        _soundService.playFailure();
+        _vibrateShort();
+
+      case FeedbackEvent.duplicateBoxNo:
+        _soundService.playFailure();
+        _vibrateShort();
+
+      case FeedbackEvent.modeSwitch:
+        _soundService.playBeep();
+    }
+  }
+
+  /// Stop any ongoing alert (e.g. when duplicate dialog is closed).
+  Future<void> stopAlert() async {
+    _soundService.stopAlert();
+  }
+
+  void _vibrateShort() {
+    Vibration.vibrate(duration: 100);
+  }
+
+  void _vibrateLong() {
+    Vibration.vibrate(duration: 200);
+  }
+
+  /// Continuous vibration pattern: 3 bursts to signal critical error.
+  void _vibratePattern() {
+    Vibration.vibrate(pattern: [0, 300, 100, 300, 100, 300]);
+  }
+
+  Future<void> dispose() async {
+    await _soundService.dispose();
+  }
+}