feat(desktop): theme the graphical layer Nord with Stylix (task 0022)

Add Stylix as a flake input and a desktop theming module that resolves a
single Nord base16 scheme across the graphical surface: GTK, Qt, cursor,
and the system monospace font, plus a static Nord wallpaper drawn as a
Polar Night gradient.

Scope the theming to the graphical layer alone. The Stylix targets for
fish (at both system and home-manager level), tmux, and nixvim stay off
so the terminal tools keep their hand-written themes.

lib.nix

@@ -51,6 +51,7 @@ let
         inputs.chaotic.nixosModules.default
         inputs.disko.nixosModules.disko
         inputs.sops-nix.nixosModules.sops
+        inputs.stylix.nixosModules.stylix
         (self + "/system.nix")
         (self + "/hosts/${hostName}")
         { networking.hostName = hostName; }