BizCore v5.1.1 - Mundo Open Context Fix (SAFE)

Purpose:
- Fixes Open Mundo redirecting to Companies when no company context is selected.
- If exactly one accessible active company exists, Mundo safely selects it and its default workspace.
- If multiple companies are available, it asks the user to select one instead of guessing.

Safety:
- No database schema changes.
- No data deletion/reset.
- No changes to sales, stock, finance, production, vendors, gateways, couriers or marketplace tables.
- Existing selected company/workspace context is preserved.

Install:
1. Extract into BizCore root and overwrite update-5.1.php if prompted.
2. Open update-5.1.php or directly mundo-open.php.
