feat(app): QR code for receive address

Adds QRCoder 1.8.0 and generates a PNG QR code in-memory each time
the receive address changes (OnReceiveAddressChanged). The bitmap is
displayed in the Receive tab inside a white-background border with
pixel-perfect scaling (BitmapInterpolationMode=None). Previous bitmap
is disposed to avoid memory leaks.

src/App/PalladiumWallet.App.csproj

@@ -22,6 +22,7 @@
       <PrivateAssets Condition="'$(Configuration)' != 'Debug'">All</PrivateAssets>
     </PackageReference>
     <PackageReference Include="CommunityToolkit.Mvvm" Version="8.4.1" />
+    <PackageReference Include="QRCoder" Version="1.8.0" />
   </ItemGroup>
 
   <ItemGroup>

src/App/ViewModels/MainWindowViewModel.cs

@@ -4,6 +4,7 @@ using System.Collections.ObjectModel;
 using System.IO;
 using System.Linq;
 using System.Threading.Tasks;
+using Avalonia.Media.Imaging;
 using Avalonia.Threading;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
@@ -15,6 +16,7 @@ using PalladiumWallet.Core.Net;
 using PalladiumWallet.Core.Spv;
 using PalladiumWallet.Core.Storage;
 using PalladiumWallet.Core.Wallet;
+using QRCoder;
 
 namespace PalladiumWallet.App.ViewModels;
 
@@ -240,6 +242,33 @@ public partial class MainWindowViewModel : ViewModelBase
     [ObservableProperty]
     private string receiveAddress = "";
 
+    /// <summary>QR code dell'indirizzo di ricezione corrente (PNG in-memory).</summary>
+    [ObservableProperty]
+    private Bitmap? receiveQr;
+
+    // Rigenera il QR ogni volta che l'indirizzo di ricezione cambia.
+    partial void OnReceiveAddressChanged(string value)
+    {
+        var previous = ReceiveQr;
+        ReceiveQr = string.IsNullOrEmpty(value) ? null : GenerateQr(value);
+        previous?.Dispose();
+    }
+
+    private static Bitmap? GenerateQr(string text)
+    {
+        try
+        {
+            using var generator = new QRCodeGenerator();
+            using var data = generator.CreateQrCode(text, QRCodeGenerator.ECCLevel.M);
+            var png = new PngByteQRCode(data).GetGraphic(8);
+            return new Bitmap(new MemoryStream(png));
+        }
+        catch
+        {
+            return null;
+        }
+    }
+
     [ObservableProperty]
     private string serverHost = "";
 

src/App/Views/MainWindow.axaml

@@ -255,6 +255,12 @@
                         <TextBlock Text="{Binding Loc[receive.next]}"/>
                         <SelectableTextBlock Text="{Binding ReceiveAddress}"
                                              FontFamily="monospace" FontSize="16"/>
+                        <Border Background="White" Padding="12" CornerRadius="6"
+                                HorizontalAlignment="Left"
+                                IsVisible="{Binding ReceiveQr, Converter={x:Static ObjectConverters.IsNotNull}}">
+                            <Image Source="{Binding ReceiveQr}" Width="220" Height="220"
+                                   RenderOptions.BitmapInterpolationMode="None"/>
+                        </Border>
                         <TextBlock Text="{Binding Loc[receive.hint]}"
                                    Foreground="Gray" FontSize="12" TextWrapping="Wrap"/>
                     </StackPanel>