Replaces the `cd frontend && npm ci` setup steps with a pnpm-flavoured
one: enable corepack (one-time), `pnpm install` from the repo root, then
`cd frontend && pnpm run <script>` for frontend scripts. misc/update.sh
also moves to `pnpm --filter mas-frontend run …`
Mention this specifically in contributing.md since CI also uses this test runner. My attempt to use `cargo test` failed due to PG connection pooling. In any case, unless there are reasons not to, we should keep close to mirroring what we do in CI.
Quentin Gliech