- Add admin-ui/index.html: web-based dashboard for monitoring hub
- Add admin-ui/README.md: setup instructions for OBSERVE_SECRET connection
- Configure nginx to serve admin-ui/ statically at /admin-ui/ location
- No separate server process needed, UI uses WebSocket to connect
- OBSERVE_SECRET provides auth layer, no nginx auth required
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
