Adds a format-check workflow that runs `pnpm run format:check` (Lerna fans
out to all packages with the script) on pushes to main/develop and all PRs.
Also adds the missing `format:check` script to the server package so Lerna
picks it up alongside the existing webapp script.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
