feat(nixos): migrate system configuration

configs/nixos/modules/desktop/utilities/userdirs.nix

@@ -0,0 +1,15 @@
+{ config, lib, ... }:
+# Declares the XDG user directories, so a tool that queries them lands in a
+# well-known folder a host can relocate from one place.
+let
+  cfg = config.modules.desktop.utilities.userdirs;
+  user = config.user.name;
+in
+{
+  options.modules.desktop.utilities.userdirs.enable = lib.mkEnableOption "XDG user directories";
+
+  config = lib.mkIf cfg.enable {
+    # Write ~/.config/user-dirs.dirs from the option defaults.
+    home-manager.users.${user}.xdg.userDirs.enable = true;
+  };
+}