feat(vercel): handle vercel previews

[omercnet]

get the hostname from searchParams so it works with vercel previews

a more robust solution would be to handle all the places where the site url is generated and fix that too

fixes https://github.com/vercel/platforms/issues/294

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[lukasburns]

Hi, thanks for this insight, In DEV or PREVIEW mode when I press for examplle a <Link href='/route'> the host does not stay, so the middleware adds de "app" host instead. How would you go about maintaining the host name when routing through the app?

Thanks!