feat: link transit registration to boxing

lib/services/scanner_service.dart

@@ -9,19 +9,24 @@ class ScanResult {
 }
 
 class ScannerService {
-  static const _eventChannel =
-      EventChannel('com.example.pad_scanner/scan');
-
-  Stream<ScanResult>? _scanStream;
+  static const _eventChannel = EventChannel('com.example.pad_scanner/scan');
+  static Stream<ScanResult>? _sharedScanStream;
 
   Stream<ScanResult> get scanResults {
-    _scanStream ??= _eventChannel
+    _sharedScanStream ??= _createSharedScanStream();
+    return _sharedScanStream!;
+  }
+
+  static Stream<ScanResult> _createSharedScanStream() {
+    return _eventChannel
         .receiveBroadcastStream()
         .map((event) => event as Map)
-        .map((event) => ScanResult(
-              barcode: event['barcode'] as String? ?? '',
-              codeType: event['codeType'] as String? ?? 'UNKNOWN',
-            ));
-    return _scanStream!;
+        .map(
+          (event) => ScanResult(
+            barcode: event['barcode'] as String? ?? '',
+            codeType: event['codeType'] as String? ?? 'UNKNOWN',
+          ),
+        )
+        .asBroadcastStream();
   }
 }