Replace Black Box with Ptyxis as the default terminal

[NN708]

This issue continues the discussion from this Discord conversation.

Feature Description

Ptyxis is becoming the default terminal in other GNOME-based distributions, such as Fedora and Ubuntu. It offers native container integration for Podman and Distrobox, providing a better user experience.

Current Limitation

When using Apx containers, Ptyxis currently logs in as root instead of the host user who launched them (chergert/ptyxis#200).

Prototype Proof

Running podman run -dt --user 1001:1001 debian:latest allows Ptyxis to log in as a normal user.

Proposed Solution

1. Replace Black Box with Ptyxis as the default terminal (installed as Flatpak). Optional: Allow users to select their preferred terminal app.
2. Update Apx to correctly set the default user.