feat: add BroadcastReceiver and EventChannel for scanner

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

android/app/src/main/kotlin/com/example/pad_scanner/MainActivity.kt

@@ -1,5 +1,45 @@
 package com.example.pad_scanner
 
+import android.content.BroadcastReceiver
+import android.content.Context
+import android.content.Intent
+import android.content.IntentFilter
 import io.flutter.embedding.android.FlutterActivity
+import io.flutter.plugin.common.EventChannel
 
-class MainActivity : FlutterActivity()
+class MainActivity : FlutterActivity() {
+
+    override fun configureFlutterEngine(flutterEngine: io.flutter.embedding.engine.FlutterEngine) {
+        super.configureFlutterEngine(flutterEngine)
+
+        EventChannel(flutterEngine.dartExecutor.binaryMessenger,
+            "com.example.pad_scanner/scan")
+            .setStreamHandler(object : EventChannel.StreamHandler {
+                private var receiver: BroadcastReceiver? = null
+
+                override fun onListen(arguments: Any?, events: io.flutter.plugin.common.EventChannel.EventSink?) {
+                    if (events == null) return
+
+                    receiver = object : BroadcastReceiver() {
+                        override fun onReceive(context: Context, intent: Intent) {
+                            val barcode = intent.getStringExtra("scannerdata") ?: return
+                            events.success(mapOf(
+                                "barcode" to barcode,
+                                "codeType" to "UNKNOWN"
+                            ))
+                        }
+                    }
+
+                    val filter = IntentFilter("com.android.server.scannerservice.broadcast")
+                    registerReceiver(receiver, filter)
+                }
+
+                override fun onCancel(arguments: Any?) {
+                    receiver?.let {
+                        unregisterReceiver(it)
+                        receiver = null
+                    }
+                }
+            })
+    }
+}