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/Services/ShopifyService.php

@@ -53,6 +53,7 @@ class ShopifyService
                     $variants[] = [
                         'id'                 => (string) $variant['id'],
                         'sku'                => $variant['sku'] ?? '',
+                        'price'              => (float) ($variant['price'] ?? 0),
                         'inventory_quantity'  => (int) ($variant['inventory_quantity'] ?? 0),
                         'product_title'      => $product['title'],
                         'option1'            => $variant['option1'] ?? '',