Refresh design of entire app

The GTK 4 port for GNOME 43 was a direct toolkit swap, only taking advantage of new widgets from gtk4/libadwaita when it was expedient to do so.

There are mockups for a fully revamped initial setup here, courtesy of @aday: https://gitlab.gnome.org/Teams/Design/os-mockups/-/blob/master/initial-setup/initial-setup.png