refactor: move graphical applications to a separate bundle

this facilitates lean clients, e.g. a laptop with only a virutal console

hosts/desktop/configuration.nix

@@ -25,6 +25,7 @@
     # keep-sorted start
     "desktop"
     "dev"
+    "gui"
     # keep-sorted end
   ]);
 

hosts/desktop/home.nix

@@ -13,6 +13,7 @@
     # keep-sorted start
     "desktop"
     "dev"
+    "gui"
     # keep-sorted end
   ]);