Add Avalonia server-host launcher (MVP scaffold)

Cross-platform C# GUI to replace the Qt/C++ Host GUI, on the same dotnet
toolchain as the server. MVP: load/save commonwealth-server.json, Start/Stop
the CommonwealthOnline.Server process (published exe, then dll, then source
run), and stream its output to a live log. Themed to the Commonwealth Online
brand (near-black + amber). Builds clean on net8.0.

Next: player list + kick/ban via the admin port, then retire the Qt host.

host/App.axaml.cs

@@ -0,0 +1,25 @@
+using Avalonia;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Markup.Xaml;
+using CommonwealthOnline.Host.ViewModels;
+using CommonwealthOnline.Host.Views;
+
+namespace CommonwealthOnline.Host;
+
+public partial class App : Application
+{
+    public override void Initialize() => AvaloniaXamlLoader.Load(this);
+
+    public override void OnFrameworkInitializationCompleted()
+    {
+        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
+        {
+            desktop.MainWindow = new MainWindow
+            {
+                DataContext = new MainWindowViewModel(),
+            };
+        }
+
+        base.OnFrameworkInitializationCompleted();
+    }
+}