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/PalladiumWallet.App.Android.csproj

@@ -23,6 +23,7 @@
 
   <ItemGroup>
     <PackageReference Include="Avalonia.Android" Version="12.0.4" />
+    <PackageReference Include="ZXing.Net" Version="0.16.9" />
   </ItemGroup>
 
   <ItemGroup>