Updated for unit-to-pcs inventory conversion.

Main behavior:
- Inventory stock saves in pcs (base unit)
- Product setup can define: 1 selected unit = N pcs
- Purchase qty entered in selected unit, stock-in saved in pcs
- Sale qty entered in selected unit, stock-out deducted in pcs
- Sale return and supplier return also convert back using base pcs logic

Important:
- Run Laravel migration after deployment to add new conversion columns.
- Migration file: database/migrations/2026_03_18_180000_add_unit_conversion_columns.php

Validation done in this environment:
- PHP syntax lint passed for all modified PHP files

Could not fully execute migrate/artisan DB flow here because the container PHP environment does not have PDO database drivers installed.
