feat(android): QR code scanner for Send address field

Camera2 + ZXing.Net 0.16.9 activity (ScannerActivity) that captures JPEG
frames at ~2.5 fps and decodes QR codes without Xamarin dependencies.
Result is returned to MainActivity via OnActivityResult/TaskCompletionSource.
PlatformServices.ScanQrAsync seam wires Android head to shared App layer;
ScanQrCommand in the Send ViewModel strips BIP21 URI scheme/query parameters.
CAMERA permission and uses-feature added to AndroidManifest.

src/App.Android/Properties/AndroidManifest.xml

@@ -5,4 +5,6 @@
 	     [Application] su MainApplication. usesCleartextTraffic resta al default
 	     (bloccato su Android 9+): il wallet preferisce comunque TLS. -->
 	<uses-permission android:name="android.permission.INTERNET" />
+	<uses-permission android:name="android.permission.CAMERA" />
+	<uses-feature android:name="android.hardware.camera" android:required="false" />
 </manifest>