desktop: replace Thunar with Nautilus, add File Roller

Thunar didn't integrate cleanly with the GNOME session. Drop the
XFCE file-manager module (thunar/xfconf/exo/tumbler) and restore
Nautilus as the native file manager: un-exclude nautilus, point the
dash favorite at org.gnome.Nautilus.desktop, remove the Thunar
desktop override, and keep gvfs (moved into gnome.nix) for mounting,
trash, and network shares. Add file-roller for archive-manager
integration in Nautilus.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/profile/nanuqsaurus.nix

@@ -16,7 +16,6 @@
     ../system/printer.nix
     ../system/impermanence.nix
     ../desktop/desktop-entries.nix
-    ../desktop/file-manager.nix
     ../desktop/gnome.nix
     ../users/admin.nix
   ];