With modal on top of side modal form, second overlay does not cover side modal

This affects confirm nav on side modal form as well as the image upload form. Demo video from https://github.com/oxidecomputer/console/pull/2328#issuecomment-2486401969 shows the approach of simply turning off the overlay for the second modal. But for image upload this feels less good because the modal is up for a while and there's a lot going on in both the upload modal and the side modal form. So the better fix would be to show the overlay but give it a higher z-index when it's on top of the side modal so that it can cover the side modal form as well. Now that I write this out, it occurs to me that it might work to simply always give the modal and its overlay that high z-index.

https://github.com/user-attachments/assets/2394caf8-64cf-4298-91ca-9c3a06b742b6

Start by reproducing the stacked-modal behavior described in the issue, including confirm navigation on a side modal form and image upload. Find the modal and overlay implementation, then verify that an overlay above a side modal covers the side modal without disrupting either workflow.