Multi-location inventory protection for product imports

Stores with multiple locations can now import products without worrying about accidentally overwriting inventory at the wrong location.

🐛 Fixes

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• Multi-location inventory safety – imports now reject Variant Inventory Qty and Variant Inventory Adjust columns when updating existing products for stores with multiple locations, preventing accidental inventory overwrites. Creating new products with these columns is still allowed and will set inventory at the default location.