Add SKU/GTIN fields, live barcode preview, and Shopify price import

- Add SKU and GTIN fields to product variant repeater
- Render EAN/UPC barcode live from GTIN input (picqer/php-barcode-generator)
- Pull variant price from Shopify API instead of defaulting to 0
- Sync sku/gtin through EditProduct and CreateProduct save logic

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Console/Commands/ShopifyImport.php

@@ -71,7 +71,7 @@ class ShopifyImport extends Command
 
                 $size = ProductSize::firstOrCreate(
                     ['product_id' => $product->id, 'packing_id' => $packing->id],
-                    ['sku' => $variant['sku'], 'shopify_variant_id' => $variant['id'], 'price' => 0],
+                    ['sku' => $variant['sku'], 'shopify_variant_id' => $variant['id'], 'price' => $variant['price']],
                 );
 
                 // Bestehende Size ohne Variant-ID nachträglich verknüpfen