Lazy drag layer(s) to the bottom of the stack if there is only 1 Artboard

With only one Artboard dragging and dropping a layer on the empty space below the stack could move them to the bottom.
Right now this is only possible with Artboards.

https://github.com/user-attachments/assets/76efa70d-a3c4-46ac-93ec-0dc51bbdd53c

Start by locating the layer-stack drag-and-drop handling and the existing behavior for dropping below a single Artboard. Compare how Artboards accept a drop in empty space, then apply the same bottom-of-stack behavior to layers when only one Artboard exists. Done means a layer can be dragged below the stack in that case and is placed at the bottom without changing multi-Artboard behavior.

Written by the indexing model from the issue text.