Task: Allow aligning non-image layers

Shapes layers have ~~feelings~~ affines too!

Specifically, I worked with someone who had a 2D+t image layer and a 2D shapes layer (see also #41), and it was not possible to use affinder to align them.

Since all napari layers natively support affine transforms, we should be able to align any layer with any other using affinder.

Start by reviewing affinder's current layer handling and the related #41 discussion, then compare how image and shapes layers expose affine transforms in napari. Done means manual correspondence-point alignment works between an image layer and a 2D shapes layer while preserving existing image-to-image behavior.

Written by the indexing model from the issue text.