feat(app): copy-to-clipboard button for receive address

Adds a "Copy" button next to the receive address in the Receive tab.
Uses Avalonia's Clipboard API (async, code-behind) and calls
NotifyAddressCopied() to show the existing addr.copied status message.
New i18n key receive.copy in all 6 languages.

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

src/App/ViewModels/MainWindowViewModel.cs

@@ -254,6 +254,9 @@ public partial class MainWindowViewModel : ViewModelBase
         previous?.Dispose();
     }
 
+    /// <summary>Feedback dopo la copia dell'indirizzo negli appunti (chiamato dal code-behind).</summary>
+    public void NotifyAddressCopied() => StatusMessage = Loc.Tr("addr.copied");
+
     private static Bitmap? GenerateQr(string text)
     {
         try