projects (frontend): add React dashboard for managers (frontend)

Add a minimal React-based dashboard (frontend/) that lists devices via API.
Files:
- frontend/package.json
- frontend/public/index.html
- frontend/src/index.jsx
- frontend/src/App.jsx
- frontend/src/api.js
- frontend/src/index.css

This gives managers a simple UI to see discovered devices and their alive state. Default API base is http://localhost:8000, configurable with REACT_APP_API_BASE.
