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/Services/ServerConfig.cs

@@ -0,0 +1,49 @@
+using System;
+using System.IO;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace CommonwealthOnline.Host.Services;
+
+// Mirrors CommonwealthOnline.Server Configuration. Keep the JSON shape aligned
+// with the server's own serializer so a config saved here loads there.
+public sealed class ServerConfig
+{
+    public string Host { get; set; } = "0.0.0.0";
+    public int Port { get; set; } = 7777;
+    public string ServerName { get; set; } = "Commonwealth Online Server";
+    public string ServerDescription { get; set; } = string.Empty;
+    public int MaxPlayers { get; set; } = 16;
+    public string LogVerbosity { get; set; } = "info";
+    public int AdminPort { get; set; } = 7779;
+    public bool EnableGnsTransport { get; set; }
+    public string? GnsBridgePath { get; set; }
+
+    private static readonly JsonSerializerOptions Options = new()
+    {
+        WriteIndented = true,
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+    };
+
+    public static ServerConfig Load(string path)
+    {
+        try
+        {
+            if (File.Exists(path))
+            {
+                return JsonSerializer.Deserialize<ServerConfig>(File.ReadAllText(path), Options)
+                       ?? new ServerConfig();
+            }
+        }
+        catch (Exception)
+        {
+            // Fall back to defaults on unreadable/invalid config.
+        }
+
+        return new ServerConfig();
+    }
+
+    public void Save(string path) =>
+        File.WriteAllText(path, JsonSerializer.Serialize(this, Options));
+}

host/Services/ServerController.cs

@@ -0,0 +1,106 @@
+using System;
+using System.Diagnostics;
+using System.IO;
+
+namespace CommonwealthOnline.Host.Services;
+
+// Launches the CommonwealthOnline.Server process, preferring a published
+// apphost, then a framework-dependent DLL, then a source-tree dotnet run.
+public sealed class ServerController
+{
+    private Process? _process;
+
+    public bool IsRunning => _process is { HasExited: false };
+
+    public event Action<string>? LogReceived;
+    public event Action<bool>? RunningChanged;
+
+    public void Start(string serverDir, string configPath)
+    {
+        if (IsRunning)
+        {
+            return;
+        }
+
+        var startInfo = ResolveLaunch(serverDir, configPath);
+        var process = new Process { StartInfo = startInfo, EnableRaisingEvents = true };
+        process.OutputDataReceived += (_, e) => Emit(e.Data);
+        process.ErrorDataReceived += (_, e) => Emit(e.Data);
+        process.Exited += (_, _) => RunningChanged?.Invoke(false);
+
+        _process = process;
+        process.Start();
+        process.BeginOutputReadLine();
+        process.BeginErrorReadLine();
+        RunningChanged?.Invoke(true);
+    }
+
+    public void Stop()
+    {
+        if (_process is { HasExited: false } process)
+        {
+            try
+            {
+                process.Kill(entireProcessTree: true);
+            }
+            catch (Exception)
+            {
+                // Process already gone or not killable; RunningChanged fires on Exited.
+            }
+        }
+    }
+
+    private void Emit(string? line)
+    {
+        if (!string.IsNullOrEmpty(line))
+        {
+            LogReceived?.Invoke(line);
+        }
+    }
+
+    private static ProcessStartInfo ResolveLaunch(string serverDir, string configPath)
+    {
+        var exeName = OperatingSystem.IsWindows()
+            ? "CommonwealthOnline.Server.exe"
+            : "CommonwealthOnline.Server";
+        var apphost = Path.Combine(serverDir, exeName);
+        var dll = Path.Combine(serverDir, "CommonwealthOnline.Server.dll");
+        var project = Path.Combine(serverDir, "CommonwealthOnline.Server.csproj");
+
+        var info = new ProcessStartInfo
+        {
+            WorkingDirectory = serverDir,
+            RedirectStandardOutput = true,
+            RedirectStandardError = true,
+            UseShellExecute = false,
+            CreateNoWindow = true,
+        };
+
+        if (File.Exists(apphost))
+        {
+            info.FileName = apphost;
+            info.ArgumentList.Add("serve");
+        }
+        else if (File.Exists(dll))
+        {
+            info.FileName = "dotnet";
+            info.ArgumentList.Add(dll);
+            info.ArgumentList.Add("serve");
+        }
+        else
+        {
+            info.FileName = "dotnet";
+            info.ArgumentList.Add("run");
+            info.ArgumentList.Add("--project");
+            info.ArgumentList.Add(project);
+            info.ArgumentList.Add("-c");
+            info.ArgumentList.Add("Release");
+            info.ArgumentList.Add("--");
+            info.ArgumentList.Add("serve");
+        }
+
+        info.ArgumentList.Add("--config");
+        info.ArgumentList.Add(configPath);
+        return info;
+    }
+}