Each size row in the product form now shows a Shopify status badge
(green ✓ Shopify / gray Lokal). For saved variants, an action button
appears: 'Zu Shopify' for local-only variants (creates via Shopify
API and stores the returned variant ID), 'Shopify sync' for linked
variants (pushes price/SKU/GTIN to Shopify).
Also fixes:
- gtin missing from ProductSize::$fillable (was silently not saved)
- shopify_product_id added to products table and stored on import
- sizes afterStateHydrated now loads ProductSize directly instead of
via the belongsToMany pivot (gets product_size_id, shopify_variant_id)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>