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/Services/PlatformServices.cs

@@ -0,0 +1,17 @@
+using System;
+using System.Threading.Tasks;
+
+namespace PalladiumWallet.App.Services;
+
+/// <summary>
+/// Seam per servizi specifici della piattaforma (Android/desktop).
+/// Il head Android imposta i delegate in OnCreate; desktop li lascia null.
+/// </summary>
+public static class PlatformServices
+{
+    /// <summary>
+    /// Apre lo scanner QR nativo e restituisce il testo raw del codice,
+    /// oppure null se l'utente annulla o lo scanner non è disponibile.
+    /// </summary>
+    public static Func<Task<string?>>? ScanQrAsync { get; set; }
+}