Prevent the user from loosing unsaved changes when navigating away

Even though this was born out of a feedback from the Visual Page Builder, we want this behavior implemented globally on all admin pages. We have something similar on Page Variants where it checks unsaved changesets but here we're also talking about navigating away to another page, not only internal actions in the same LiveView.

It should warn about unsaved changes and give the option to Save before leaving the page.

Start by inspecting the existing Page Variants unsaved-changeset check and the admin page navigation flow. Verify the behavior across admin pages: leaving with unsaved changes should warn and offer Save, while navigation without changes should proceed.

Written by the indexing model from the issue text.