[FEATURE] Use XDG_PICTURES_DIR for default screenshot location

[dommcdev]

Submission checklist

• I have searched existing issues and confirmed this has not been requested before.
• I have checked existing pull requests for similar changes.

Feature type

New functionality

Feature summary

Currently the default dir for screenshots is hardcoded to $HOME/Pictures, with an option to change it in settings. I think it would make more sense to use XDG_PICTURES_DIR (and/or XDG_SCREENSHOTS_DIR, but I think it is less commonly set) instead.

Motivation / use case

This would allow noctalia to work better ootb for those who change XDG_PICTURES_DIR from the default of $HOME/Pictures.

Proposed solution

Default to XDG_PICTURES_DIR, with a fallback to $HOME/Pictures if it isn't set for some reason.

Alternatives considered

No response

References / related projects

Grimblast (a hyprland screenshot tool) has this behavior
https://github.com/hyprwm/contrib/blob/3dcbce715ae8b93107fa8632db15bf976862a573/grimblast/grimblast#L47

Additional context

(This would also apply to the official screen recorder plugin via XDG_VIDEOS_DIR).