desktop: rename gnome.nix → desktop.nix; switch to nemo

- Rename settings/gnome.nix to settings/desktop.nix (file no longer
  has anything to do with GNOME)
- Replace nautilus with nemo — starts fast, has a hamburger menu,
  no Tracker3/Mutter D-Bus dependencies
- Exclude thunar (pulled in by XFCE) via environment.xfce.excludePackages
- Remove nautilus Tracker dconf workaround (no longer needed)
- Update $mod+E keybind and common.nix import accordingly

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

common.nix

@@ -9,7 +9,7 @@
     ./hosts/FredOS-Mediaserver.nix
     
     # Generic settings #
-    ./settings/gnome.nix
+    ./settings/desktop.nix
     ./settings/hyprland.nix
     ./settings/stylix.nix
     ./settings/locale.nix