Tie stylix base16Scheme to catppuccin/nix flavor

When running stylix alongside catppuccin/nix, stylix still needs a base16 scheme for apps catppuccin/nix has no port for (compositor borders, gtk, browsers). Rather than hard-coding a flavor, set base16Scheme = "${pkgs.base16-schemes}/share/themes/catppuccin-${config.catppuccin.flavor}.yaml"; base16-schemes ships catppuccin's official base16 ports for all four flavors. Then changing catppuccin.flavor moves both systems together. This needs the catppuccin module imported wherever the stylix module is, including standalone home-manager configs. To test changes to a flake input's local clone before pushing, use nix eval --override-input <name> path:/local/clone.