fix: Custom fields delete bug — usunięcie wszystkich pól produktu nie działało

Dodano hidden marker custom_field_name_present w formularzu edycji produktu.
Zmieniono warunek w ProductRepository z array_key_exists('custom_field_name')
na array_key_exists('custom_field_name_present') — jQuery .serialize() pomijał
klucz pustej tablicy gdy wszystkie pola usunięte. Test jednostkowy dodany.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/Unit/Domain/Product/ProductRepositoryTest.php

@@ -1292,4 +1292,25 @@ class ProductRepositoryTest extends TestCase
 
         $this->assertFalse($result);
     }
+
+    public function testSaveCustomFieldsDeletesAllWhenEmpty(): void
+    {
+        $mockDb = $this->createMock(\medoo::class);
+
+        $mockDb->expects($this->once())
+            ->method('delete')
+            ->with(
+                $this->equalTo('pp_shop_products_custom_fields'),
+                $this->equalTo(['id_product' => 55])
+            );
+
+        $mockDb->expects($this->never())->method('insert');
+        $mockDb->expects($this->never())->method('update');
+
+        $repository = new ProductRepository($mockDb);
+
+        $method = new \ReflectionMethod(ProductRepository::class, 'saveCustomFields');
+        $method->setAccessible(true);
+        $method->invoke($repository, 55, [], [], []);
+    }
 }