Can't cancel stitching from GUI

Bug report

Summary

At the end of a scan, or when stitching from the scan tab, there's no option to cancel an ongoing stitch. Especially when we prioritise reliability over speed, that can lead to a long lock up of the scan, as we can't close the stitching modal to start a new scan either

Configuration

I'm using:

v3, Pi 4 2GB

Steps to reproduce

Run a large scan with stitching enabled, or stitch a finished scan from the scan tab

/cc @rwb27