Serve the frontend's config.js with the API password

The bundled frontend gets a token with API_PASSWORD from config.js. The server now
generates config.js from its own config, so setting API_PASSWORD on the server
container is enough.

Dockerfile

@@ -18,8 +18,6 @@ RUN --mount=type=cache,target=/go/pkg/mod --mount=type=cache,target=/root/.cache
 
 # Stage 2: Frontend static files (the same for every platform)
 FROM --platform=$BUILDPLATFORM ${FRONTEND_IMAGE} AS frontend
-# Empty API_HOSTNAME makes the frontend use relative paths
-RUN echo "window.__RUNTIME_CONFIG__ = { API_HOSTNAME: '' };" > /usr/share/nginx/html/config.js
 
 # Stage 3: Final image. No RUN steps here, so no emulation is needed for other architectures.
 FROM alpine:3.22