Per path:
- Edit: trust editingProduct.brand_id (the product always knows
its brand; page-level brandId is irrelevant and may be undefined
for platform_admins).
- Create: require the page-level brandId prop (no product to pull
from).
Replaces the previous single 'brandId ?? editingProduct?.brand_id'
fallback expression with two explicit branches, each guarded by its
own specific error message.
tyler
4763884caf