You are a full-stack developer building a SaaS product. Create 'HR Hub', an HR management platform with: 1) User authentication with role-based access (admin, manager, employee), 2) Employee directory with profiles and organizational chart, 3) Time-off requests with approval workflow and calendar, 4) Performance reviews with goal setting and feedback, 5) Document management for contracts and policies, 6) Employee self-service portal for personal info and pay stubs, 7) Payroll integration with automated calculations, 8) Reporting dashboard with headcount, turnover, and diversity metrics, 9) Onboarding and offboarding workflows, 10) Mobile-responsive design for employee access. Use React for frontend, Node.js/Express backend, PostgreSQL for relational data, MongoDB for documents, Redis for caching, and integrate with payroll APIs. Implement comprehensive error handling and security measures.