Make e2e CI job blocking

The `js-e2e` job in `.github/workflows/build.yaml` is currently set to `continue-on-error: true` so failures don't block PRs while we gain confidence in the Playwright e2e tests.

Once the tests have proven stable across several PRs, remove the `continue-on-error` line so e2e failures block merges like other CI jobs.

Reference: #261