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/ViewModels/MainWindowViewModel.cs

@@ -0,0 +1,97 @@
+using System.Collections.ObjectModel;
+using System.IO;
+using Avalonia.Threading;
+using CommonwealthOnline.Host.Services;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+
+namespace CommonwealthOnline.Host.ViewModels;
+
+public partial class MainWindowViewModel : ObservableObject
+{
+    private const int MaxLogLines = 2000;
+
+    private readonly ServerController _controller = new();
+    private readonly string _serverDir = System.IO.Directory.GetCurrentDirectory();
+    private readonly string _configPath =
+        Path.Combine(System.IO.Directory.GetCurrentDirectory(), "commonwealth-server.json");
+
+    [ObservableProperty] private string _serverName;
+    [ObservableProperty] private string _host;
+    [ObservableProperty] private int _port;
+    [ObservableProperty] private int _maxPlayers;
+    [ObservableProperty] private int _adminPort;
+    [ObservableProperty] private string _logVerbosity;
+    [ObservableProperty] private bool _enableGnsTransport;
+    [ObservableProperty] private bool _isRunning;
+    [ObservableProperty] private string _statusText = "Stopped";
+
+    public ObservableCollection<string> Log { get; } = new();
+
+    public string[] VerbosityOptions { get; } = { "error", "warning", "info", "debug" };
+
+    public MainWindowViewModel()
+    {
+        var config = ServerConfig.Load(_configPath);
+        _serverName = config.ServerName;
+        _host = config.Host;
+        _port = config.Port;
+        _maxPlayers = config.MaxPlayers;
+        _adminPort = config.AdminPort;
+        _logVerbosity = config.LogVerbosity;
+        _enableGnsTransport = config.EnableGnsTransport;
+
+        _controller.LogReceived += line =>
+            Dispatcher.UIThread.Post(() => Append(line));
+        _controller.RunningChanged += running =>
+            Dispatcher.UIThread.Post(() =>
+            {
+                IsRunning = running;
+                StatusText = running ? "Running" : "Stopped";
+                StartCommand.NotifyCanExecuteChanged();
+                StopCommand.NotifyCanExecuteChanged();
+            });
+    }
+
+    private void Append(string line)
+    {
+        Log.Add(line);
+        while (Log.Count > MaxLogLines)
+        {
+            Log.RemoveAt(0);
+        }
+    }
+
+    private ServerConfig CurrentConfig() => new()
+    {
+        ServerName = ServerName,
+        Host = Host,
+        Port = Port,
+        MaxPlayers = MaxPlayers,
+        AdminPort = AdminPort,
+        LogVerbosity = LogVerbosity,
+        EnableGnsTransport = EnableGnsTransport,
+    };
+
+    [RelayCommand]
+    private void Save() => CurrentConfig().Save(_configPath);
+
+    [RelayCommand(CanExecute = nameof(CanStart))]
+    private void Start()
+    {
+        Save();
+        Append($"[host] starting server on {Host}:{Port}...");
+        _controller.Start(_serverDir, _configPath);
+    }
+
+    private bool CanStart() => !IsRunning;
+
+    [RelayCommand(CanExecute = nameof(CanStop))]
+    private void Stop()
+    {
+        Append("[host] stopping server...");
+        _controller.Stop();
+    }
+
+    private bool CanStop() => IsRunning;
+}