Enhance DiscourseGraphPanel with drag-and-drop functionality and state management (#366)

- Implemented drag-and-drop capabilities for nodes and relations within the DiscourseGraphPanel.
- Utilized useMemo for performance optimization in panel item generation and unique relations.
- Introduced drag state management using useAtom for better control over drag events.
- Refactored event handlers for pointer interactions to improve user experience.
- Updated UI to reflect dragging state with a preview of the dragged item.

Author: mdroidian