Upgrade NixOS to 25.11 and fix Jovian module organization

- Update flake inputs from 25.05 to 25.11 (nixpkgs, home-manager, nix-darwin)
- Remove Jovian compatibility shim that's no longer needed
- Move SteamOS configuration to only import in nix-deck machine
- Fixes jovian module not found error during nixos-rebuild

machines/nix-deck/configuration.nix

@@ -2,6 +2,7 @@
 {
   imports = [
     ./hardware-configuration.nix
+    ../../roles/desktop/steamos.nix
   ];
 
   roles = {