feat: update admin and learning apps to use React 19 and Ant Design 6, add loading fallback components

- Updated admin app to React 19 and Ant Design 6, ensuring both apps maintain separate routers, API clients, and build outputs.
- Introduced `LoadingFallback` component for better user experience during loading states in both apps.
- Refactored routes in the learning app to utilize lazy loading for improved performance.
- Added linting and type checking commands for better code quality assurance.
- Updated documentation to reflect changes in project structure and usage instructions.