Critical fix for £0 order bug:
- When users select a variant and click 'Add to Basket', the confirmvar: callback triggers HandleConfirmVariant
- This method was passing variantId: null to AddItem(), causing cart items to have no variant and price £0
- Now looks up selected variant by name, extracts its ID, and passes it to cart
- Added logging to track which variant is being used
- Also includes CSV variant conversion utility and sample fixed import file
