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/Models/Product.php

@@ -22,8 +22,7 @@ class Product extends Model
     public function sizes()
     {
         return $this->belongsToMany(Packing::class, 'products_sizes')
-                    ->withPivot('price')
-                    ->withPivot('description')
+                    ->withPivot('price', 'description', 'sku', 'gtin')
                     ->withTimestamps();
     }