Run the merged frontend vitest suite on every frontend-touching PR so the
tests don't rot. Mirrors the #1032 backend gate: per-job dorny/paths-filter
(new `frontend` output, `frontend/**`), node:20 + npm cache on the lockfile,
`npm ci --legacy-peer-deps --no-audit --no-fund` (exact frontend/Dockerfile
flags), then `npm run test`. Additive — backend-tests job unchanged; no
top-level paths filter so frontend-only PRs already trigger the workflow.
Making frontend-tests a required check is a follow-up in the Forgejo UI.
Refs #999.
