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));
+}