Rename nextjs.yml to build.yml

The workflow file was named nextjs.yml from when the frontend was a
Next.js app. The workflow actually builds the frontend (React/Vite),
installs backend deps, and tests the Docker build — nothing Next.js
specific. Rename to build.yml to accurately reflect its purpose.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: gkorland