Initial commit: Client Doc docs Server Tools

Co-authored-by: Cursor <cursoragent@cursor.com>

Server/Auth/DuplicateUsernameException.cs

@@ -0,0 +1,10 @@
+using System;
+
+namespace XWorld.Server.Auth
+{
+    public sealed class DuplicateUsernameException : Exception
+    {
+        public DuplicateUsernameException(string username)
+            : base("username already taken: " + username) { }
+    }
+}