fix deprecated package names and gtk4 theme warning

- protonvpn-gui → proton-vpn
- xfce.thunar → thunar
- set gtk.gtk4.theme = null to adopt new HM 26.05 default

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

settings/desktop.nix

@@ -19,7 +19,7 @@
     services.xserver.desktopManager.xfce.enable = true;
 
     # Thunar is pulled in by XFCE but we use Nemo instead.
-    environment.xfce.excludePackages = [ pkgs.xfce.thunar ];
+    environment.xfce.excludePackages = [ pkgs.thunar ];
 
     # Flatpak for ad-hoc app installs via Bazaar
     services.flatpak.enable = true;
@@ -52,6 +52,7 @@
       # Minimal titlebars — stylix manages the GTK theme; we layer our
       # headerbar shrink on top via stylix.targets.gtk.extraCss.
       gtk.enable = true;
+      gtk.gtk4.theme = null;
 
       dconf.settings."org/nemo/window-state".start-with-menu-bar = false;