astro integration should respect base path

[stefanprobst]

trafficstars

when deploying a site with a base path (e.g. www.example.com/my-site), the requests to the /api/keystatic endpoints currently don't take the base path setting into account, i.e. keystatic will try to talk to www.example.com/api/keystatic/github/refresh-token instead of www.example.com/my-site/api/keystatic/github/refresh-token