chore(scanner): add duplicate-ID cooldown for rapid scan stability

- Ignore repeated processing of the same HW ID within a short 2s window
- Prevent duplicate dialogs/inserts during very fast repeated captures
- Keep continuous scanner loop responsive while reducing accidental re-triggers

lib/screens/scan_tab.dart

@@ -14,11 +14,14 @@ class ScanTab extends StatefulWidget {
 
 class ScanTabState extends State<ScanTab> {
   static const _activeCollectionPrefKey = 'active_collection_id';
+  static const _duplicateCooldown = Duration(seconds: 2);
 
   bool _isBusy = false;
   List<Collection> _collections = [];
   Collection? _selectedCollection;
   bool _loadingCollections = true;
+  String? _lastProcessedHwId;
+  DateTime? _lastProcessedAt;
 
   @override
   void initState() {
@@ -285,6 +288,15 @@ class ScanTabState extends State<ScanTab> {
   }
 
   Future<bool> _processHwId(String hwId) async {
+    final now = DateTime.now();
+    if (_lastProcessedHwId == hwId &&
+        _lastProcessedAt != null &&
+        now.difference(_lastProcessedAt!) < _duplicateCooldown) {
+      return true;
+    }
+    _lastProcessedHwId = hwId;
+    _lastProcessedAt = now;
+
     final collection = _selectedCollection;
     if (collection == null) return false;