[DevOps]: Add frontend pull-request checks

[tomvothecoder]

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