Menu imports: circular reference detection and clearer errors

We’ve improved menu imports to be safer and easier to troubleshoot.

🔧 Improvements

• Circular reference detection – identifies loops in nested menu items and halts import with a clear message (read more)
• Better parent linking – prevents attaching a menu item to itself by title during import

🐛 Fixes

• Consistent error code – surfaces MENU007 when a circular menu structure is detected (see docs)