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/Views/MainWindow.axaml.cs

@@ -0,0 +1,11 @@
+using Avalonia.Controls;
+
+namespace CommonwealthOnline.Host.Views;
+
+public partial class MainWindow : Window
+{
+    public MainWindow()
+    {
+        InitializeComponent();
+    }
+}