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/utilities/Validator.h

@@ -9,10 +9,15 @@
 #include<string>
 #include<algorithm>
 #include<cctype>
+#include "Map.h"
+#include "User.h"
 
 namespace util
 {
 	bool isPhoneNumberValid(const std::string&);
 	bool isEmailValid(const std::string&);
 	bool isPasswordValid(const std::string&);
+	bool isUsernameDuplicate(const std::string&, const util::Map<std::string, User*>&);
+	bool isPhoneDuplicate(const std::string&, const util::Map<std::string, User*>&);
+	bool isEmailDuplicate(const std::string&, const util::Map<std::string, User*>&);
 }