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>

settings/hyprland.nix

@@ -218,7 +218,7 @@ in
           # Mirror the GNOME bindings so muscle memory carries over
           bind = [
             "$mod, T, exec, $term"
-            "$mod, E, exec, nautilus"
+            "$mod, E, exec, nemo"
             "$mod, R, exec, $menu"
             "$mod, Q, killactive"
             "$mod SHIFT, E, exit"