fix: Proxy not picked up on Windows (#85443)
Proxy is not being picked up during prod on Windows. It is because of
two reasons:
1. `isAtConventionLevel` condition was false as the dir was `\\` in
Windows, but was checking with explicit `/`.
2. `page` value passed to `getStaticInfoIncludingLayouts` function was
in `\\proxy` but was expected to be `/proxy`, which is later used for
`isProxy` condition.
Therefore, the given paths needed to be normalized.
Current behavior:
https://github.com/vercel/next.js/actions/runs/18872127141/job/53853048061?pr=85443#step:34:160
Closes NEXT-4756
