Fix css preload when serving from proxied subpaths (#9822)
* Fix assets when serving from subpath
* add changeset
---------
Co-authored-by: gradio-pr-bot <gradio-pr-bot@users.noreply.github.com>
Co-authored-by: Abubakar Abid <abubakar@huggingface.co>
Co-authored-by: pngwn <hello@pngwn.io>
