fix: Prefer $XDG_CONFIG_HOME to $HOME/.config on Linux

[Ratizux]

Flatpak apps should store config to $XDG_CONFIG_HOME according to Flatpak docs.

Current implementation uses $HOME/.config, which is on tmpfs in Flatpak by default if no host directory access is granted (according to Flatpak docs). Such in the case, PlumeImpactor would lost config after restart.

Use $XDG_CONFIG_HOME, which is persistent in Flatpak sandboxes, fall back to $HOME/.config if the former does not exist.

Fixes: #153

A quick fix for current Flatpak users is flatpak override --user --persist=.config dev.khcrysalis.PlumeImpactor, which maps .var/app/dev.khcrysalis.PlumeImpactor/.config to $HOME/.config so to make it persistent.

[claration]

Thank you! I'm not very well versed in linux so I was never sure if using $HOME is the best way to do it.