Add GitHub Actions workflow for linting and production build

## Summary
Create CI automation to run lint and production build on pull requests and pushes.

## Details
- Adds GitHub Actions workflow file
- Runs `npm run lint`
- Runs `npm run build`
- Prevents regressions before merge

## Acceptance Criteria
- [ ] Workflow triggers on PR and push
- [ ] Lint passes successfully
- [ ] Build passes successfully