use jovian-nixos + fixup niri display session handling

etcnixos/system-yarn.nix

@@ -18,6 +18,7 @@
     inputs.nixos-hardware.nixosModules.common-cpu-amd-zenpower
     inputs.impermanence.nixosModules.impermanence
     inputs.disko.nixosModules.disko
+    inputs.jovian-nixos.nixosModules.default
   ];
 
   fileSystems."/media/games" = {
@@ -58,4 +59,17 @@
   systemd.services.lactd.wantedBy = [ "multi-user.target" ];
 
   systemd.services.lactd.serviceConfig.ExecStartPre = "${lib.getExe pkgs.bash} -c \"sleep 3s\"";
+
+  jovian = {
+    devices.steamdeck.enable = false;
+    steam = {
+      enable = true;
+      autoStart = true;
+      desktopSession = "niri";
+      user = username;
+    };
+  };
+
+  # Disable gamescope from common.nix to avoid conflict with jovian-nixos
+  programs.gamescope.enable = lib.mkForce false;
 }