ci(webapp): add lint and build steps to webapp CI workflow

[alikhere]

Summary

The webapp CI workflow only ran tests — a PR that broke the build or introduced lint errors would ship green. Added two steps
after the existing test step:

• Lint runs npm run lint with continue-on-error: true because there are pre-existing lint errors in the repo unrelated to any individual PR. This way lint output is visible in CI without blocking merges until those are cleaned up separately.
• Build runs npm run build and must pass. A broken build now blocks merges.

How to test

• Open a PR against main and check the Run NPM Tests on Pull Request workflow run
• Lint step should appear and report results (may show warnings/errors from pre-existing issues) without failing the job
• Build step should appear and pass; introduce a syntax error to verify it fails and blocks the merge

FIxes #181