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.
Each held one default.nix, so the directory added a level to descend without
grouping anything. They are now lib.nix and system.nix beside flake.nix, which
reads as the flake's own scaffolding.
The shared base config's reference to the shared secrets file was relative to
the directory it sat in, so it moves with the file.
Verified to build the same system toplevel as before the move.
