Improve adaptive behaviour

[mxmvncnt]

Currently, the Back button is handled based on if the main Leaflet was folded when a vault item has been clicked. This causes problems as if the app is open (both columns), and one clicks a vault item and then resizes the app, there is no Back button. This also causes problems when someone clicks a vault item with the main leaflet folded (mobile view), as the button will appear, but it will not be removed when the window is expanded again.

note for when I get around to this: https://gnome.pages.gitlab.gnome.org/libadwaita/doc/main/adaptive-layouts.html, I think it needs a second headerbar with separator to add a button back onto