feat: Add support for listing and restoring Flatpak applications.

[hthienloc]

This PR brings several related changes:

• List and restore Flatpak applications when backing up/restoring software selections.
  • Query flatpak list for installed apps.
  • Include fp:… entries in package lists and handle them specially.
  • Add new helper methods (_install_flatpaks, etc.) to install Flatpaks during restore.
• Update UI and logic for toggles & package lists to handle Flatpaks.
• Version bump to 2.4.8 (changelog entry) reflecting the new feature.
• Minor code cleanup around subprocess imports.

The feature has been tested locally: backups now include Flatpaks and restore correctly, and the UI remains responsive.

Why?
#98 #99

[SethyM12]

I tested this new commit on my machine and I did not find any issues. Flatpaks were included in the output list file and then were able to be restored along with system packages.