BizCore v6.2.1 - Mundo UI Polish

UI-only safe update.
- Premium cart layout with product thumbnail, order summary and trust cues.
- Cleaner Mundo admin dashboard with KPI cards and grouped operations.
- Mobile responsive refinements.
- CSS cache version moved to 6.2.1.

No database migration or data reset.
Install: extract into BizCore root with overwrite, then open /update-6.2.1.php.
