refactor(stasis): take the package from nixpkgs

nixpkgs carries stasis 1.5.1, so the pinned input and pkgs/stasis.nix go. It
ships no unit and has no NixOS module, so modules/services.nix keeps declaring
one; its actions run as `sh -lc` and it spawns niri and pactl directly, hence
the unit path.

stasis prefers a user config over /etc and generates its own when none exists,
so moonarch's is seeded into the home directory instead of only reaching /etc.

modules/services.nix

@@ -96,17 +96,21 @@
     };
   };
 
-  # Upstream ships a unit in the Arch package but not in the tarball, so it is
-  # declared here. Mirrors /usr/lib/systemd/user/stasis.service.
+  # nixpkgs ships the package but no module, and the package carries no unit of
+  # its own, so it is declared here. Mirrors /usr/lib/systemd/user/stasis.service.
   systemd.user.services.stasis = {
     description = "Stasis Wayland Idle Manager";
     partOf = [ "graphical-session.target" ];
     after = [ "graphical-session.target" ];
     wantedBy = [ "graphical-session.target" ];
+    # stasis runs its actions as `sh -lc <command>` and spawns niri and pactl
+    # directly, all resolved against this PATH. pulseaudio is here for pactl
+    # alone; it stays off systemPackages, see modules/desktop.nix.
+    path = with pkgs; [ bash swaylock brightnessctl niri pulseaudio ];
     serviceConfig = {
       Type = "simple";
-      ExecStart = "${lib.getExe moonarchPkgs.stasis}";
-      ExecReload = "${lib.getExe moonarchPkgs.stasis} reload";
+      ExecStart = "${lib.getExe pkgs.stasis}";
+      ExecReload = "${lib.getExe pkgs.stasis} reload";
       Restart = "on-failure";
       RestartSec = 2;
     };