Steam Deck support

The Steam Deck is a Linux console that runs SteamOS. This operating system is read-only under the hood, so paths like `/var` are not writable, and can be overwritten with system updates. Despite `pacman` not being usable by default because of this, `python3` is an included package, so running the quickstart works like a charm out of the box.

However, it'd be great if there were systemd user configs (and any other recommended settings) tailored for running copyparty on startup in SteamOS!