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>

database/migrations/2026_09_12_100000_add_shopify_product_id_to_products.php

@@ -0,0 +1,22 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('products', function (Blueprint $table) {
+            $table->string('shopify_product_id')->nullable()->after('id');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('products', function (Blueprint $table) {
+            $table->dropColumn('shopify_product_id');
+        });
+    }
+};