backlight: use brightnessctl instead of removed light binary

nixpkgs 26.05 dropped the 'light' package. Switch BacklightService to brightnessctl and bundle it in the wrapper PATH. Bump flake nixpkgs to nixos-26.05.

nix/package.nix

@@ -8,6 +8,7 @@
   nova-shaders,
   nova-plugin,
   glib,
+  brightnessctl,
 }:
 let
   shellSrc = lib.cleanSourceWith {
@@ -53,6 +54,7 @@ symlinkJoin {
         lib.makeBinPath [
           nova-stats
           glib
+          brightnessctl
         ]
       }
   '';