feat: add ScannerService with EventChannel

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

lib/services/scanner_service.dart

@@ -0,0 +1,27 @@
+import 'dart:async';
+import 'package:flutter/services.dart';
+
+class ScanResult {
+  final String barcode;
+  final String codeType;
+
+  ScanResult({required this.barcode, required this.codeType});
+}
+
+class ScannerService {
+  static const _eventChannel =
+      EventChannel('com.example.pad_scanner/scan');
+
+  Stream<ScanResult>? _scanStream;
+
+  Stream<ScanResult> get scanResults {
+    _scanStream ??= _eventChannel
+        .receiveBroadcastStream()
+        .map((event) => event as Map)
+        .map((event) => ScanResult(
+              barcode: event['barcode'] as String? ?? '',
+              codeType: event['codeType'] as String? ?? 'UNKNOWN',
+            ));
+    return _scanStream!;
+  }
+}