feat(desktop): add screen brightness controls

Bind XF86MonBrightnessUp/Down to brightnessctl in the repeat-while-held
block, mirroring the volume keys. A new brightness module installs
brightnessctl's udev rule and adds the user to the video group so the
backlight can be dimmed without root.

modules/desktop/desktop.nix

@@ -25,5 +25,6 @@ in
     modules.desktop.theming.enable = lib.mkDefault true;
     modules.desktop.waybar.enable = lib.mkDefault true;
     modules.desktop.audio.enable = lib.mkDefault true;
+    modules.desktop.brightness.enable = lib.mkDefault true;
   };
 }