Add optional FNSKU validation in Scan and Pack

Add an optional Scan and Pack validation setting for shipments with mixed barcode workflows. When enabled, the scanner should accept valid FNSKU labels, accept manufacturer barcodes only when the SKU is configured as commingled, and show an error such as "Item requires an FNSKU label" when a unit still requires FNSKU labeling. This would reduce packing errors on high-SKU shipments where items may have FNSKU labels, manufacturer barcodes, and custom identifiers.