Update frontend Dockerfile prod stage from Caddy static to Node.js
running Astro SSR server. Configure caddy-docker-proxy labels to
route /api/* to backend and everything else to frontend on the same
domain. Support empty PUBLIC_API_URL for same-origin API calls.
Switch from query-param routing (/notebook/?id=X) to Astro dynamic
routes (/notebook/rc-lowpass-filter). Add @astrojs/node adapter with
output: 'server' for on-demand route handling.
Fix formatEng/formatAxisValue crash on null values passed by uPlot
axis tick formatters. Add CORS origin for port 4322.
