feat(menu): printer setup — system-config-printer in the System submenu

A `nomarchy-menu printers` entry opens system-config-printer (the CUPS
admin GUI), mirroring the bluetooth/blueman pattern: the printing service
ships the package via environment.systemPackages and the menu execs it,
self-gated on the system-config-printer binary so the entry appears only
when nomarchy.services.printing is on. No direct keybind (rare one-off).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/nixos/services.nix

@@ -75,7 +75,8 @@ in
 
     printing.enable = lib.mkEnableOption ''
       CUPS printing with Avahi/mDNS, so network printers are auto-discovered
-      (add vendor drivers via `services.printing.drivers`)'';
+      (add vendor drivers via `services.printing.drivers`); the menu's
+      System ▸ Printers entry opens the system-config-printer GUI'';
 
     openrgb.enable = lib.mkEnableOption ''
       the OpenRGB daemon and GUI for controlling RGB lighting on peripherals
@@ -246,6 +247,9 @@ in
         nssmdns4 = true;
         openFirewall = true;
       };
+      # The CUPS admin GUI — the menu's System ▸ Printers entry execs it
+      # (self-gated on this binary), so it ships with the printing service.
+      environment.systemPackages = [ pkgs.system-config-printer ];
     })
 
     (lib.mkIf cfg.openrgb.enable {