fix(mackup): update backup path and enhance configuration handling

- Corrected the backup path in the `_in_backup` function to reflect the new structure under `mackup/`.
- Improved error handling in `ssr::mackup::configure` to skip restore if the configuration file is not updated, providing user feedback.
- Updated `ssr::sync` to conditionally execute backup based on configuration validity, enhancing robustness.
- Revised documentation in configuration examples to clarify the new Mackup directory structure and its implications for users.

bin/ssr-restore

@@ -90,7 +90,10 @@ ssr::icloud::prefetch "$mackup_dir"
 # ssr::mackup::restore already calls ssr::icloud::wait internally.
 _ssr_mackup_restore_step() {
     ssr::mackup::ensure_installed
-    ssr::mackup::configure "$mackup_dir"
+    ssr::mackup::configure "$mackup_dir" || {
+        ssr::warn "Skipping mackup restore — ~/.mackup.cfg was not updated (see messages above)."
+        return 1
+    }
     ssr::mackup::restore
 }