Drop the Vercel Sync admin page and its backend (internal/httpapi/admin/vercel,
webui/src/features/vercel), unregister its routes, remove the nav item and
locale keys, and delete the Vercel Go entrypoint api/index.go. Deployment is
docker-image only.
The inert internal stream-prepare plumbing (chat/vercel_stream.go and the
__stream_prepare branches) is left in place; it never executes without the
Vercel Node bridge and removing it would touch concurrency-sensitive lease
code in the core streaming handlers.