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/MainActivity.cs

@@ -1,11 +1,12 @@
+using System.Threading.Tasks;
 using Android.App;
+using Android.Content;
 using Android.Content.PM;
+using Android.OS;
 using Avalonia.Android;
 
 namespace PalladiumWallet.Mobile;
 
-// Activity di avvio. La configurazione dell'app (tipo App, font) è nella
-// MainApplication (AvaloniaAndroidApplication<App>). Qui basta il launcher.
 [Activity(
     Label = "Palladium Wallet",
     Theme = "@style/MyTheme.NoActionBar",
@@ -13,4 +14,24 @@ namespace PalladiumWallet.Mobile;
     ConfigurationChanges = ConfigChanges.Orientation | ConfigChanges.ScreenSize | ConfigChanges.UiMode)]
 public class MainActivity : AvaloniaMainActivity
 {
+    internal const int ScanRequestCode = 9001;
+    internal static TaskCompletionSource<string?>? ScanTcs;
+    internal static MainActivity? Current;
+
+    protected override void OnCreate(Bundle? savedInstanceState)
+    {
+        base.OnCreate(savedInstanceState);
+        Current = this;
+    }
+
+    protected override void OnActivityResult(int requestCode, Result resultCode, Intent? data)
+    {
+        base.OnActivityResult(requestCode, resultCode, data);
+        if (requestCode == ScanRequestCode)
+        {
+            var text = resultCode == Result.Ok ? data?.GetStringExtra(ScannerActivity.ResultKey) : null;
+            ScanTcs?.TrySetResult(text);
+            ScanTcs = null;
+        }
+    }
 }