feat(desktop): resolve media output paths through XDG user-dirs

Add a modules.desktop.userdirs module that declares the XDG user directories,
and route the recorder and screenshot tools through xdg-user-dir so their
output folders (Videos/Recordings, Pictures/Screenshots) follow one relocatable
source instead of a hardcoded $HOME path.

modules/desktop/userdirs.nix

@@ -0,0 +1,16 @@
+{ 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.userdirs;
+  user = config.user.name;
+in
+{
+  options.modules.desktop.userdirs.enable = lib.mkEnableOption "XDG user directories";
+
+  config = lib.mkIf cfg.enable {
+    # enable writes ~/.config/user-dirs.dirs from the option defaults, which
+    # xdg-user-dir then reads.
+    home-manager.users.${user}.xdg.userDirs.enable = true;
+  };
+}