Implement review fixes

Changes:

 - Strengthened UserManagementService::updateUserDetails by checking duplicates only when email/phone are changed, preventing false errors
 - Updated AdminMenu::viewStockLevels header text from "View Stock Level" to "View Stock Levels" for consistency
 - Cleaned up CustomerMenu::updateDetails by removing unused user list retrieval and improving header/message formatting

Trenser.VehicleServiceSystem/Trenser.VehicleServiceSystem/views/AdminMenu.cpp

@@ -159,7 +159,7 @@ void AdminMenu::viewStockLevels()
     util::clear();
 	auto inventoryItems = m_controller.getInventoryItems();
 	bool hasActiveItems = false;
-	std::cout << "View Stock Level" << std::endl;
+	std::cout << "View Stock Levels" << std::endl;
 	if (inventoryItems.isEmpty())
 	{
 		std::cout << "No items found in Inventory.\n";

Trenser.VehicleServiceSystem/Trenser.VehicleServiceSystem/views/CustomerMenu.cpp

@@ -143,7 +143,6 @@ Return type: void
 void CustomerMenu::updateDetails()
 {
 	std::string email, phone;
-    util::Map<std::string, const User*> userList = m_controller.getUsers();
 	util::clear();
     std::cout << "Update Details\n";
 	std::cout << "Enter new email: ";