[DevOps]: Add frontend pull-request checks

### Task Details

The repository has backend pull-request checks in `/home/runner/work/simboard/simboard/.github/workflows/backend-ci.yml`, but there is no equivalent frontend PR workflow.

This leaves frontend changes without automated pull-request validation even though `/home/runner/work/simboard/simboard/frontend/package.json` already defines `lint`, `type-check`, and `ci` scripts.

Add a GitHub Actions workflow that runs on pull requests affecting `frontend/**` and relevant shared config, and validates the frontend with the existing project commands.

### Steps to Complete

1. Add a frontend CI workflow for pull requests to `main`
2. Scope triggers to `frontend/**` and relevant shared config files
3. Install frontend dependencies with pnpm
4. Run the existing frontend validation command(s)
5. Confirm the workflow appears as a required PR check

### Additional Notes

Related evidence:
- `/home/runner/work/simboard/simboard/frontend/package.json`
- `/home/runner/work/simboard/simboard/.github/workflows/backend-ci.yml`