feature(CORS): Remove CORS proxy for R2 parquet downloads in Pyodide

[szemyd]

Summary

This PR removes the CORS proxy layer for parquet file downloads in Pyodide/WASM environments. Parquet files are now downloaded directly from presigned R2 URLs, which have CORS headers configured on the R2 buckets to allow cross-origin GET requests from the browser.

Key Changes

• Simplified download flow: Removed the proxy URL construction logic that routed requests through {base_url}/data/proxy
• Direct R2 access: Parquet files are now fetched directly from presigned URLs using pyfetch(url, method="GET")
• Removed unnecessary headers: Eliminated the _to_js_headers() conversion since presigned URLs carry authentication in query parameters (X-Amz-*) and don't require additional headers
• Updated documentation: Module docstring and function docstring now reflect that CORS is configured on R2 buckets rather than proxied
• Updated tests: Test cases now verify direct presigned URL fetching instead of proxy-based routing

Implementation Details

• Presigned URLs from R2 include authentication via query parameters, eliminating the need for custom headers or proxy intermediaries
• The change reduces latency by removing an extra hop through the proxy endpoint
• CORS configuration on the R2 buckets handles cross-origin requests directly from browser environments

https://claude.ai/code/session_011sUKbmutP1bnrtk6yJgUUX