Fix niri config discovery, polkit path, wallpaper stack

- NIRI_CONFIG points niri at /etc/xdg; without it niri writes its own
  default config, so none of the Moonarch settings applied.
- config.kdl spawns the polkit agent by FHS path, provided via tmpfiles
  instead of diverging the config shared with Arch.
- waypaper was missing entirely, hence no wallpaper. It invokes swww,
  which the maintained awww provides under a different name.

modules/services.nix

@@ -88,17 +88,7 @@
     };
   };
 
-  # Polkit authentication agent for the graphical session.
-  systemd.user.services.polkit-gnome-authentication-agent-1 = {
-    description = "polkit-gnome authentication agent";
-    partOf = [ "graphical-session.target" ];
-    after = [ "graphical-session.target" ];
-    wantedBy = [ "graphical-session.target" ];
-    serviceConfig = {
-      Type = "simple";
-      ExecStart = "${pkgs.polkit_gnome}/libexec/polkit-gnome-authentication-agent-1";
-      Restart = "on-failure";
-      RestartSec = 1;
-    };
-  };
+  # The polkit agent is not a service here: niri spawns it via
+  # spawn-at-startup in defaults/xdg/niri/config.kdl. Only the FHS path it
+  # references has to exist — see modules/desktop.nix.
 }