Reflect already-allocated locations in the outbound shipment location picker

When selecting locations on an outbound shipment, locations already allocated to another open (not yet shipped) outbound shipment still appear as available in the picker. They only disappear once the earlier shipment is shipped. With multiple open shipments this is confusing and risks double-allocating the same location's stock. Request: flag or hide locations whose stock is already committed to another open outbound shipment.