use jovian-nixos + fixup niri display session handling

etcnixos/common.nix

@@ -6,6 +6,7 @@
   system,
   hostname,
   inputs,
+  niri-package,
   ...
 }:
 {
@@ -33,6 +34,9 @@
 
   services.kmscon.enable = true;
 
+  # Add niri to display manager session packages
+  services.displayManager.sessionPackages = [ niri-package ];
+
   # Gamescope configuration for Steam Deck-like experience
   programs = {
     gamescope = {
@@ -45,26 +49,6 @@
     };
   };
 
-  services.greetd = {
-    enable = true;
-    settings = {
-      default_session = {
-        command = "${lib.getExe pkgs.tuigreet} --sessions /etc/xdg/wayland-sessions/ --time";
-        user = username;
-      };
-      terminal.vt = lib.mkForce 2;
-    };
-  };
-
-  environment.etc."xdg/wayland-sessions/niri-session.desktop".text = ''
-    [Desktop Entry]
-    Name=Niri Session
-    Comment=Niri Wayland compositor
-    Exec=niri-session
-    Type=Application
-    DesktopNames=niri
-  '';
-
   system.activationScripts = {
     # FIX: https://github.com/NixOS/nix/issues/2982
     "profile-channel-dummy".text = ''
@@ -97,6 +81,9 @@
     ];
   };
 
+  # allow unfree packages for jovian-nixos
+  nixpkgs.config.allowUnfree = true;
+
   # kernel options
   boot = {
     kernelPackages = pkgs.linuxPackages_latest;