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/views/UserInterface.cpp

@@ -136,6 +136,7 @@ void UserInterface::registerCustomer()
 {
 	std::string username, name, email, phone, password;
 	util::clear();
+	std::cout << "Register Customer\n";
 	std::cout << "Enter username: ";
 	util::read(username);
 	std::cout << "Enter name: ";
@@ -165,6 +166,6 @@ void UserInterface::registerCustomer()
 		return;
 	}
 	m_controller.createCustomer(username, name, password, email, phone);
-	std::cout << "Registration is successful";
+	std::cout << "Registration is successfull";
 	util::pressEnter();
 }