Add Shopify publish/sync per product variant

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>

app/Models/ProductSize.php

@@ -9,7 +9,7 @@ class ProductSize extends Model
 {
     use HasFactory;
     protected $table = 'products_sizes';
-    protected $fillable = ['product_id', 'packing_id', 'sku', 'shopify_variant_id', 'price', 'description'];
+    protected $fillable = ['product_id', 'packing_id', 'sku', 'shopify_variant_id', 'gtin', 'price', 'description'];
 
     public function product() {
         return $this->belongsTo(Product::class);