desktop: Add X-Purism-FormFactor
So the app is detected as mobile-friendly on Phosh.
See also https://gitlab.gnome.org/GNOME/Incubator/snapshot/-/merge_requests/134
So the app is detected as mobile-friendly on Phosh.
See also https://gitlab.gnome.org/GNOME/Incubator/snapshot/-/merge_requests/134