fix: remove dead root vercel.json now that Root Directory is set to frontend

Root Directory is now configured as `frontend` in Vercel project
settings, so Vercel can no longer see (or read) this file — it can't
access anything outside its Root Directory. Zero-config Next.js
detection now runs directly against frontend/package.json,
frontend/pnpm-lock.yaml and frontend/.npmrc, which are already fully
self-contained. This replaces the fragile dual pnpm --dir install/build
command chain, which was the actual cause of the install-time
"ENOENT ... frontend/frontend" failure (a doubled path from combining
that manual --dir with the platform's own directory handling).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BRDH4c1gieXvgcMREtazSR

Author: christabel888