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/services/UserManagementService.cpp

@@ -113,15 +113,21 @@ void UserManagementService::updateUserDetails(const std::string& userID, const s
 	{
 		throw std::runtime_error("User does not exist!\n");
 	}
-	if (util::isEmailDuplicate(email, usersMap))
-	{
-		throw std::runtime_error("Email already exists!\n");
-	}
-	if (util::isPhoneDuplicate(phone, usersMap))
-	{
-		throw std::runtime_error("Phone number already exists!\n");
-	}
 	User* user = usersMap.getValueAt(index);
+	if (email != user->getEmail())
+	{
+		if (util::isEmailDuplicate(email, usersMap))
+		{
+			throw std::runtime_error("Email already exists!\n");
+		}
+	}
+	if (phone != user->getPhone())
+	{
+		if (util::isPhoneDuplicate(phone, usersMap))
+		{
+			throw std::runtime_error("Phone number already exists!\n");
+		}
+	}
 	user->setEmail(email);
 	user->setPhone(phone);
 }