Stop baking VITE_API_URL into the Vite build so one image can
run against any backend. Container entrypoint
(docker-entrypoint.d/30-frontend-config.sh, auto-run by the
nginx image) writes /usr/share/nginx/html/config.js from the
VITE_API_URL container env at every start; index.html loads
it before the bundle, and agent.ts prefers
window.__APP_CONFIG__ with the build-time env as fallback so
local dev and vite preview behave exactly as before.
public/config.js ships an empty placeholder for non-container
builds. Changing backends is now a restart with new env, no
rebuild.
