Boot into graphical.target so greetd starts

greetd is WantedBy=graphical.target, but an install without a desktop
defaults to multi-user.target and never activates it.

modules/greetd.nix

@@ -35,6 +35,10 @@ in
     };
   };
 
+  # greetd is WantedBy=graphical.target. An installation without a desktop
+  # boots into multi-user.target, where the unit never starts.
+  systemd.defaultUnit = "graphical.target";
+
   # The greeter session spawns `moongreet` by name from niri-greeter.kdl,
   # so it has to resolve in PATH.
   environment.systemPackages = [ moonarchPkgs.moongreet ];