Add admin console for managing users; default account is admin

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -10,7 +10,7 @@ python3 server.py adduser NAME   # create a player account
 python3 server.py serve          # --upstream http://HOST:8000 --port 8080
 ```
 
-Then open `http://<server-ip>:8080` and sign in. Other commands: `users`, `passwd NAME`, `deluser NAME`.
+Then open `http://<server-ip>:8080` and sign in. The first account (and the `PLAYER_USER` default in Docker) is an admin. Admins manage users at `/admin`. Other commands: `users`, `passwd NAME`, `deluser NAME`.
 
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