fix: Add wallpaper.png

A wallpaper is required for Stylix to work, so I added one with base00
as the background color and the NixOS logo as the foreground image.

Credit for the logo goes to the original author who licensed it under
CC-BY: https://releases.nixos.org/nix-dev/2016-October/021876.html

configuration.nix

@@ -13,9 +13,8 @@
   networking.useHostResolvConf = true;
   services.udisks2.enable = true;
   security.pam.services.swaylock = { };
-  # stylix.image = ./background.jpg;
+  stylix.image = ./wallpaper.png;
   stylix.polarity = "dark";
-  # stylix.base16Scheme = ./ayu-dark.yaml;
   stylix.base16Scheme = "${pkgs.base16-schemes}/share/themes/monokai.yaml";
 
   stylix.fonts = {