cosmic-session: Install dconf profile
As of this change upstream, distro packages should provide a dconf profile for COSMIC. Without one, GTK applications will not pick up COSMIC settings – for example, whether or not to show the minimize/maximize buttons in window titlebars. Additionally, failing to provide a COSMIC profile file will cause the session to have a read-only dconf store.
There is a note that the upstream dconf profile is specific to Pop!_OS, so it might be better to create an Arch-specific one, but I've tested the change and it appears to be working as expected.