docs: document api.ui.admin

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015M9wLCvRaA5jj8cCZN6wFg

Author: b0bbywan

README.md

@@ -318,6 +318,7 @@ api:
   port: 8018
   ui:
     enabled: true
+    # admin: ":8021"  # optional link to an admin web app, same forms as service urls
   sse:
     enabled: true
   cors: