Improve the macOS uninstall process

The macOS uninstaller script (uninstall.sh) leaves the GUI running and fails to remove instances due to authentication (tested on MacOS but also reported on Windows).

A recommendation would be to update `uninstall.sh `to stop all Multipass processes before removing files and remove the unnecessary `multipass delete --purge --all` call (Virtualbox has been deprecated).

Start with uninstall.sh and inspect how the macOS uninstall flow handles running Multipass processes, authentication, and instance removal. Update the script according to the issue's recommendations, then verify that the GUI is stopped, instances can be removed, and the unnecessary VirtualBox purge call is gone.

Written by the indexing model from the issue text.