Refactor user management: Extract IUserService and UserService, consolidate user registration, authentication, and profile management. Fix memory leaks in ApiClient, enhance connection management, and improve error handling in AuthController and UsersController. Update IRC command handling to utilize IUserService for user operations.

src/EchoHub.Server/Program.cs

@@ -115,6 +115,7 @@ while (true)
 
         // ── Chat Service + Broadcasters ─────────────────────────────────────
         builder.Services.AddSingleton<IChatBroadcaster, SignalRBroadcaster>();
+        builder.Services.AddSingleton<IUserService, UserService>();
         builder.Services.AddSingleton<IChannelService, ChannelService>();
         builder.Services.AddSingleton<IChatService, ChatService>();