Updated Flatpak commands to use the --user flag, ensuring all operations (install, uninstall, update) affect only the current user's Flatpaks. Also updated the Zen browser Flatpak ID and improved error handling during installation.
Flatpak management has been refactored from Home Manager and nix-flatpak to a new system/flatpak.nix module, now imported in system/common.nix. This centralizes Flatpak enablement and declarative app management at the system level, simplifying configuration and maintenance.
