This PR addresses the issue where some clients (like the R bindings) were receiving corrupt/compressed data because the proxy was serving cached gzip/brotli responses to clients that didn't support them.

What I changed in

app-next/src/app/api/proxy-file/route.ts

1 - Forwarded Accept-Encoding: The proxy now passes the client's original encoding preferences to the upstream server. This ensures the upstream only sends compressed data if the client actually asked for it.
2 - Added Vary: Accept-Encoding: I added this header to the proxy's response. This tells CDNs and intermediate caches to store separate versions of the file based on the client's encoding support, preventing "cross-client" cache contamination.
3 - Passed through Content-Encoding: If the upstream does return compressed data, the proxy now correctly forwards that header so the client knows it needs to decompress the response.

Testing done:

1 - Ran a full npm run build to make sure the route compiles correctly.
2 - Verified with tsc --noEmit and eslint (0 errors).
3 - Double-checked the logic to ensure we aren't adding unnecessary headers to non-binary files.
Fixes #367