services: Add gamemode-service-type.

* modules/rosenthal/services/games.scm
(<gamemode-configuration>): New data type.
(gamemode-account-service, gamemode-etc-service): New procedures.
(gamemode-service-type): New variable.

modules/rosenthal/services/games.scm

@@ -0,0 +1,62 @@
+;;; SPDX-License-Identifier: GPL-3.0-or-later
+;;; Copyright © 2026 Hilton Chain <hako@ultrarare.space>
+
+(define-module (rosenthal services games)
+  ;; Utilities
+  #:use-module (guix gexp)
+  #:use-module (guix packages)
+  #:use-module (guix records)
+  #:use-module (rosenthal utils file)
+  ;; Guix System
+  #:use-module (gnu system accounts)
+  #:use-module (gnu system shadow)
+  ;; Guix System - services
+  #:use-module (gnu services)
+  #:use-module (gnu services configuration)
+  #:use-module (gnu services dbus)
+  ;; Guix packages
+  #:use-module (gnu packages linux)
+  #:export (gamemode-service-type
+            gamemode-configuration))
+
+(define-maybe gexp)
+
+(define-configuration/no-serialization gamemode-configuration
+  (gamemode
+   (file-like gamemode)
+   "@code{gamemode} package to use.")
+  (config
+   maybe-gexp
+   ""))
+
+(define (gamemode-account-service config)
+  (list (user-group
+          (system? #t)
+          (name "gamemode"))))
+
+(define gamemode-etc-service
+  (match-record-lambda <gamemode-configuration>
+      (gamemode config)
+    `(("gamemode.ini"
+       ,(if (maybe-value-set? config)
+            (ini-file "gamemode.ini" config)
+            (file-append (package-source gamemode) "/example/gamemode.ini")))
+      ("security/limits.d/10-gamemode.conf"
+       ,(file-append gamemode "/etc/security/limits.d/10-gamemode.conf")))))
+
+(define gamemode-service-type
+  (service-type
+    (name 'gamemode)
+    (extensions
+     (list (service-extension profile-service-type
+                              (compose list gamemode-configuration-gamemode))
+           (service-extension account-service-type
+                              gamemode-account-service)
+           (service-extension etc-service-type
+                              gamemode-etc-service)
+           (service-extension dbus-root-service-type
+                              (compose list gamemode-configuration-gamemode))
+           (service-extension polkit-service-type
+                              (compose list gamemode-configuration-gamemode))))
+    (default-value (gamemode-configuration))
+    (description "Set up Guix System environment for gamemode.")))