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.

modules/desktop/theming.nix

@@ -0,0 +1,65 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# Nord theming for the graphical layer, driven from one base16 scheme by Stylix.
+let
+  cfg = config.modules.desktop.theming;
+  user = config.user.name;
+
+  # A single static Nord wallpaper, drawn as a vertical gradient across the
+  # Polar Night shades so it needs no committed binary and no network fetch.
+  wallpaper = pkgs.runCommand "nord-wallpaper.png" { } ''
+    ${pkgs.imagemagick}/bin/magick \
+      -size 2560x1440 gradient:'#2E3440'-'#3B4252' \
+      "$out"
+  '';
+in
+{
+  options.modules.desktop.theming.enable =
+    lib.mkEnableOption "Nord theming of the graphical layer via Stylix";
+
+  config = lib.mkIf cfg.enable {
+    stylix = {
+      enable = true;
+
+      # Nord is a dark scheme.
+      # The polarity steers Stylix's contrast choices.
+      polarity = "dark";
+      base16Scheme = "${pkgs.base16-schemes}/share/themes/nord.yaml";
+
+      image = wallpaper;
+
+      cursor = {
+        package = pkgs.bibata-cursors;
+        name = "Bibata-Modern-Ice";
+        size = 24;
+      };
+
+      fonts.monospace = {
+        package = pkgs.jetbrains-mono;
+        name = "JetBrains Mono";
+      };
+
+      # fish is also installed at system level, so its theming has a target
+      # here of its own.
+      targets.fish.enable = false;
+    };
+
+    home-manager.users.${user} = {
+      # Stylix drives the cursor through home-manager's pointer-cursor config,
+      # whose generation must be switched on explicitly.
+      home.pointerCursor.enable = true;
+
+      # nvim, tmux, and fish are configured through home-manager and keep their
+      # own hand-written themes, so their Stylix targets stay off.
+      stylix.targets = {
+        fish.enable = false;
+        tmux.enable = false;
+        nixvim.enable = false;
+      };
+    };
+  };
+}