feat: add wlsunset home-manager module for Wayland night light

Replace the manual wlsunset package + commented exec-once with a proper
home-manager services.wlsunset module tied to hyprland-session.target.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nixos/desktop.nix

@@ -5,6 +5,7 @@ makeEnable config "myModules.desktop" true {
     ./fonts.nix
     ./hyprland.nix
     ./keyd.nix
+    ./wlsunset.nix
   ];
 
   assertions = [