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/Console/Commands/ShopifyImport.php

@@ -58,17 +58,29 @@ class ShopifyImport extends Command
 
                 $product = Product::firstOrCreate(
                     ['title' => $variant['product_title']],
-                    ['recipes_id' => null, 'description' => $variant['product_description']],
+                    [
+                        'recipes_id'         => null,
+                        'description'        => $variant['product_description'],
+                        'shopify_product_id' => $variant['shopify_product_id'],
+                    ],
                 );
                 if ($product->wasRecentlyCreated) {
                     $stats['products']++;
-                    // Collect for image download after transaction
                     $newProducts[$product->id] = [
                         'product'    => $product,
                         'image_urls' => $variant['image_urls'],
                     ];
-                } elseif (empty($product->description) && ! empty($variant['product_description'])) {
-                    $product->update(['description' => $variant['product_description']]);
+                } else {
+                    $updates = [];
+                    if (empty($product->description) && ! empty($variant['product_description'])) {
+                        $updates['description'] = $variant['product_description'];
+                    }
+                    if (empty($product->shopify_product_id)) {
+                        $updates['shopify_product_id'] = $variant['shopify_product_id'];
+                    }
+                    if ($updates) {
+                        $product->update($updates);
+                    }
                 }
 
                 $size = ProductSize::firstOrCreate(