The rclone web GUI uses the Prefer header for sync/copy operations,
but the CORS middleware's preflight response did not include it in
Access-Control-Allow-Headers. This caused the browser to block
remote GUI copy requests with a CORS policy error:
Request header field prefer is not allowed by
Access-Control-Allow-Headers in preflight response.
Add 'Prefer' to the allowed headers list so remote GUI
access works correctly for file copy operations.
Fixes#9614
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
