BizCore v5.5 - Mundo Seller Product Center

SAFE ADDITIVE UPDATE. Does not delete or reset existing BizCore/Mundo records.

Install:
1. Extract ZIP into the current BizCore domain root and overwrite files.
2. Login as Super Admin.
3. Open /update-5.5.php
4. Click Install / Repair Mundo v5.5.
5. Seller login -> My Products to submit listings.
6. BizCore -> Mundo Ecommerce -> Vendor Products to review/approve listings and commission overrides.

Added:
- Seller Product Center
- Seller price / stock / SKU / category / brand / image URL
- Pending approval on new/edited seller listings
- Product archive (non-destructive)
- Public seller shop product grid for approved listings
- Premium responsive Mundo UI polish
- Seller dashboard Manage Products entry

Note: v5.5 is the product-management foundation. Checkout routing/settlement logic remains under BizCore control and can be extended in the next stage.
