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/brightness.nix

@@ -0,0 +1,23 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# Screen backlight control via brightnessctl.
+let
+  cfg = config.modules.desktop.brightness;
+  user = config.user.name;
+in
+{
+  options.modules.desktop.brightness.enable = lib.mkEnableOption "brightnessctl backlight control";
+
+  config = lib.mkIf cfg.enable {
+    environment.systemPackages = [ pkgs.brightnessctl ];
+
+    # The udev rule chgrps each backlight's brightness node to `video` and adds
+    # group write, so a member can dim the panel without root.
+    services.udev.packages = [ pkgs.brightnessctl ];
+    users.users.${user}.extraGroups = [ "video" ];
+  };
+}