kayla: hide fred from the SDDM greeter

hosts/FredOS-Kayla.nix

@@ -8,6 +8,9 @@
     services.displayManager.sddm = {
       enable = true;
       wayland.enable = true;
+      # fred exists on every host for admin/SSH, but this is her machine —
+      # keep the greeter to one face. Still reachable over SSH and on a TTY.
+      settings.Users.HideUsers = "fred";
     };
     services.desktopManager.plasma6.enable = true;