refactor: improve layout structure and responsiveness of the demo page

- Reorganized header, code panel, event stream, DAG, and details sections into CSS grid
- Enhanced desktop layout with defined grid areas and columns for better alignment
- Added responsive styles for tablet and mobile views, stacking components vertically
- Removed redundant code and comments, clarified layout styles for maintainability
- Updated class names and inline styles to support flexible and consistent layout behavior

Author: jumski