fix(scripts): add next-server to serve.sh cleanup trap (#1162)

The cleanup() trap kills "next dev" and "next start" but not
"next-server". Since "next start" forks a "next-server" child
process, killing the parent may leave the child running as a
zombie, holding port 3000. The startup teardown block (line 35)
already handles this, but the Ctrl+C / SIGTERM trap did not.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>