Adds UIBaseURL (env PROXY_UI_URL), advertised separately from BaseURL
for deployments where the UI is reached on a public domain while build
machines hit a Docker network alias for the package endpoints. Defaults
to BaseURL when unset.
Consumers:
- Logged alongside base_url at startup.
- <link rel="canonical"> and og:url / og:title / og:site_name in every
UI page, omitted when UIBaseURL is empty.
- Banner on the install guide when UIBaseURL differs from BaseURL,
clarifying that the URLs in the snippets are the package endpoint and
that the UI itself lives elsewhere.
Docs call out that the proxy serves UI and package endpoints on the same
listener, so reverse proxies fronting the UI publicly must restrict the
public route to PathPrefix(/ui) to avoid exposing /npm, /pypi, etc.
nginx and Traefik examples both show the path-split pattern.
