Complete web interface overhaul with authentication and modern UI

- Implemented comprehensive user authentication system (login/logout/register)
- Added beautiful landing page for non-authenticated users
- Created user dashboard with statistics and quick actions
- Built modern, responsive UI with Inter font and professional styling
- Added session management and user context throughout
- Implemented token-based usage limits and tracking
- Created job history page with detailed status tracking
- Added user settings page with preferences and API token management
- Integrated flash messaging for user feedback
- Added navigation headers with consistent branding
- Implemented role-based access control decorators
- Added comprehensive error handling and user feedback

The web interface now provides a complete, professional user experience with authentication, modern design, and full functionality for the multi-process Video AI system.