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.

flake.nix

@@ -40,6 +40,13 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
 
+    # Themes the graphical layer from one base16 scheme.
+    # Follows our nixpkgs so it themes the same package set the host builds.
+    stylix = {
+      url = "github:danth/stylix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
     # Agent-ergonomic CLI for Gitea, with its home-manager module wiring in the
     # Claude Code context where that harness is present.
     gitea-axi = {