[napari workflows] Review 2D & 3D image handling in napari workflows

[jluethi]

See #403 and #398

I covered some scenarios, but I'm not sure whether the "calculate a label image in napari workflows based on an actual 2D image OME-Zarr intensity image" would work as expected. I think it should (and should just be saved as an actual 2D image), but to be tested a bit further. Plus test the different interactions with expected_dimensions and if there are weird edge cases there

I should review this and check whether the napari workflows flow can be simplified.

[jluethi]

Also review the napari workflow tests and see if we can increase coverage for the different 2D vs 3D scenarios