Deliver Apx GUI as a Flatpak

[NN708]

Now that Apx GUI is available on Flathub (Vanilla-OS/apx-gui#87) and Distrobox is installed on the host system (Vanilla-OS/core-image#116), we can deliver Apx GUI as a Flatpak.

This approach offers several benefits: a smaller root image, independent release cycles, and potentially improved security.

Users could run flatpak run --command=apx org.vanillaos.ApxGUI if they still prefer using the Apx CLI.