Fix: add duplicate user validation and update register customer UI

- add username duplicate validation
- add email duplicate validation
- add phone duplicate validation
- move duplicate checks to Validator utility
- add Register Customer header
- update registration success message

Fixes #1737

Trenser.VehicleServiceSystem/Trenser.VehicleServiceSystem/services/UserManagementService.cpp

@@ -19,7 +19,7 @@ Date:19-May-2026
 #include "User.h"
 #include "UserManagementService.h"
 #include "Vector.h"
-
+#include "Validator.h"
 
 /*
 Function: ensureAdminExists
@@ -74,16 +74,18 @@ void UserManagementService::createUser(const std::string& username, const std::s
 	PaymentManagementService paymentManagementService;
 	ServiceManagementService serviceManagementService;
 	auto& usersMap = m_dataStore.getUsers();
-	int index = usersMap.findIf(
-		[&](const std::string&, User* user)
-		{
-			return user->getUserName() == username;
-		}
-	);
-	if (index != -1)
+	if (util::isUsernameDuplicate(username, usersMap))
 	{
 		throw std::runtime_error("Username already exists");
 	}
+	if (util::isEmailDuplicate(email, usersMap))
+	{
+		throw std::runtime_error("Email already exists");
+	}
+	if (util::isPhoneDuplicate(phone, usersMap))
+	{
+		throw std::runtime_error("Phone already exists");
+	}
 	User* newUser = Factory::getObject<User>(username, password, name, phone, email, type);
 	usersMap.insert(newUser->getId(), newUser);
 	paymentManagementService.attach(newUser);