fix(css): apply server.origin to public file urls in CSS (fix #18457)

[SalvatoreT]

Fixes #18457

Vite's CSS plugin rewrites url() references during dev by resolving them to absolute URLs with the dev server's origin hardcoded. The server.origin config exists but is only applied in some code paths (module assets in templates and JS imports) but not in others (CSS url() rewriting path).

Similar to @kajic on #18457, I have a base for creating the HTML and a different one for the Vite server.

This has turned out to be a problem for me because I serve HTML generated by a Vite SSR server from a server in front of the Vite dev server. In short, a server at http://127.0.0.1:8000/ requests HTML from Vite running at http://127.0.0.1:5173/, and the fronting server returns the HTML it got from Vite to the browser. At this point, all URLs in the HTML are relative to http://127.0.0.1:8000/, instead of being relative to http://127.0.0.1:5173/, so all static assets on the page fail to load.

This change just lest the server.origin also get applied to the CSS files too.

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[sapphi-red]

Instead of a unit test here, would you add a test to playground/backend-integration?

Also it would be nice to have tests for non-public assets.