users/guest: init and add steam

users/guest/common/optional/graphical/steam.nix

@@ -0,0 +1,30 @@
+{
+  pkgs,
+  ...
+}:
+{
+  home.packages = with pkgs; [
+    gamescope
+    steam
+  ];
+
+  systemd.user.services.steam-big-picture = {
+    Unit = {
+      Description = "Steam Big Picture in Gamescope";
+      After = [
+        "graphical.target"
+        "default.target"
+      ];
+    };
+    Service = {
+      ExecStart = ''
+        ${pkgs.gamescope}/bin/gamescope --rt --backend drm --steam -- \
+          ${pkgs.steam}/bin/steam -pipewire-dmabuf -tenfoot
+      '';
+      Restart = "always";
+    };
+    Install = {
+      WantedBy = [ "default.target" ];
+    };
+  };
+}