Decide whether to keep cell drag functionality for unified cells

[coderabbitai]

Context

In PR #50, we added undo/redo functionality and row-level drag support. During testing, we discovered that the cell drag option for unified cells needs further consideration.

Decision Points

• Should we keep the cell drag option for unified cells?
• If yes, we need to add a proper drop zone for it to make the UX clear and consistent

Background

The current implementation has drag-and-drop working but may need refinement for unified cells specifically. This came up during investigation of websocket closure issues in CI tests.

References

• PR: [FEAT] Add undo/redo support with history panel #50
• Comment: [FEAT] Add undo/redo support with history panel #50 (comment)

Requested by @Avni2000

[Avni2000]

Like everything else, it is entirely possible to just throw this into the settings and forget about it, but I worry this feature will become unmaintained if that happens. Furthermore, first use on firefox 147 was really strange b/c it didn't "drag" correctly - that will need to be made smoother/some alternative should be made.