spd-2026-2/chat-socket-tcp ? reverse-engineered prompt

Build me a simple Java chat app that works with a client and a server over TCP.

I want the server to let multiple people connect at the same time, each using a nickname, and reject duplicate nicknames. The chat should support sending messages to everyone, sending a private message to just one person, and showing who is online. When someone joins or leaves, the other users should see a system message. Use JSON for the messages, one message per line, so it’s easy for the client and server to talk to each other.

The client should be a small Swing app where I can type the server IP, port, and my nickname, then connect and chat. It should keep listening for new messages in the background while I use the interface. Use localhost and port 6000 by default if that helps. If you need to look up current docs online, feel free to do that.