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/desktop/gnome.nix

@@ -30,6 +30,9 @@ in
   services.displayManager.gdm.enable = true;
   services.desktopManager.gnome.enable = true;
 
+  # Virtual filesystem backend for Nautilus: mounting, trash, network shares.
+  services.gvfs.enable = true;
+
   services.gnome.core-developer-tools.enable = false;
   services.gnome.games.enable = false;
 
@@ -41,7 +44,7 @@ in
           "org/gnome/shell" = {
             disable-user-extensions = false;
             favorite-apps = [
-              "thunar.desktop"
+              "org.gnome.Nautilus.desktop"
               "org.gnome.Ptyxis.desktop"
               "com.ranfdev.DistroShelf.desktop"
               "io.github.kolunmi.Bazaar.desktop"
@@ -94,6 +97,8 @@ in
     pkgs.gnome-extension-manager
     pkgs.mission-center
     pkgs.papirus-icon-theme
+    pkgs.file-roller
+    pkgs.unrar
     pkgs.gnomeExtensions.all-in-one-clipboard
     pkgs.gnomeExtensions.alphabetical-app-grid
     pkgs.gnomeExtensions.app-hider
@@ -129,7 +134,6 @@ in
     pkgs.gnome-system-monitor
     pkgs.gnome-weather
     pkgs.loupe
-    pkgs.nautilus
     pkgs.papers
     # gnome-connections
     pkgs.showtime