Urgent: Fix Shopify Variant Logic Uminex Theme (Shopify partners/experts only!)

My store runs on the Uminex Shopify theme, but its variant selector behaves unpredictably: unavailable option combinations disappear or grey out incorrectly and the Add to Cart button often shows the wrong state as soon as there is more than one variant for a product.

I need the smooth, intuitive behaviour exactly like you find in the Shopify Dawn theme.

Your job is to recreate Dawn’s variant-handling logic inside Uminex so that:
• Every option remains visible and clickable at all times.
• Sold-out combinations are simply crossed out, never hidden or disabled.
• The Add to Cart button flips instantly between “Add to Cart” and “Sold Out” with every change.

All existing customisations—price formatting, SKU and EAN displays, product labels, metafields and anything else already present—must stay exactly as they are.

You need to check the complete theme and repair and optimize all neccessary files!!!

Please supply a clean, well-commented Liquid + JavaScript solution that drops into the current theme without disturbing other files. Test it thoroughly on desktop and mobile;

Once installed I should see Dawn-style variant handling across the entire catalogue with zero regression elsewhere.