BizCore v5.4 - Mundo Seller Portal Foundation

Adds:
- Public Mundo "Become a Seller" application
- BizCore Seller Applications review screen
- Approve / Reject / Request Changes workflow
- Approval creates/activates BizCore vendor profile with seller-level default commission
- Product-level commission overrides remain supported by v5.2 and take priority over seller default
- Separate Mundo DB seller account + shop shell
- Seller activation/login/dashboard foundation
- Public seller shop page

Install:
1. Extract into BizCore root, overwrite when asked.
2. Open /update-5.4.php as Super Admin.
3. Click Install / Repair Mundo v5.4.
4. Test /mundo/become-seller.php.

This is additive. It does not delete existing BizCore, Mundo, inventory, sales, finance, vendor, order, or storefront records.
