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/Program.cs

@@ -0,0 +1,16 @@
+using System;
+using Avalonia;
+
+namespace CommonwealthOnline.Host;
+
+internal static class Program
+{
+    [STAThread]
+    public static void Main(string[] args) =>
+        BuildAvaloniaApp().StartWithClassicDesktopLifetime(args);
+
+    public static AppBuilder BuildAvaloniaApp() =>
+        AppBuilder.Configure<App>()
+            .UsePlatformDetect()
+            .LogToTrace();
+}