Refactor DiagramEditor for modularity and enhanced link handling:

- Extract `NavigationControls`, `BackgroundGrid`, and `LinkingPreview` into separate components.
- Update `LinkComponent` to dynamically compute paths with `diagramUtils` and improve waypoint handling.
- Improve keyboard interactions: enable zoom, deletion, and arrow key-based step movement.
- Add context menu for empty space to facilitate step creation of various types.
- Enhance code readability and maintainability by removing redundant logic and simplifying methods.