feat(steam): add system profile for Gamescope with Jovian

Introduces a dedicated NixOS system profile for Steam, integrating Jovian-NixOS for Gamescope.
This includes:
- New 'jovian' flake input and module.
- New 'steam' profile using 'jovian' as a specialisation.
- Opens port 8050 in the tower firewall.
- Sets Alacritty to open maximized in Niri.

modules/home/niri/default.nix

@@ -225,6 +225,7 @@ in {
       window-rules = [
         {
           matches = [{app-id = "Alacritty";}];
+          open-maximized = true;
           open-on-workspace = "1";
         }
         {

modules/nixos/jovian/default.nix

@@ -0,0 +1,17 @@
+{
+  inputs,
+  lib,
+  ...
+}: {
+  imports = [inputs.jovian.nixosModules.default];
+
+  jovian.steam = {
+    enable = true;
+    autoStart = true; # Jovian gère lui même le login
+    user = "marc";
+    desktopSession = "niri"; # "Switch to Desktop" relance niri
+  };
+
+  # autoStart est incompatible avec un autre display manager
+  services.displayManager.sddm.enable = lib.mkForce false;
+}