User module | Register feature

### User Story

As a user, I should be able to log into the application using a valid email and password. If the email or password is incorrect, the application will display an error message indicating that the login credentials are invalid. Specifically, if the email is invalid, the error message will be "Invalid email address." If the email is valid but the password is incorrect, the error message will be "Invalid password."

### Register Task
- [x] Implement user registration functionality in the Service Layer.
- [x] Create a Data Access Object (DAO) to save user information to the database.
- [x] Write JUnit Test Cases to verify the user registration process:
- [x] Test successful user registration.
- [x] Test registration with an invalid email format.
- [x] Test registration with a weak password.
- [x] Test registration with a missing name.
- [x] Test registration with an invalid phone number format.
- [x] Ensure that all test cases are passing successfully.
- [x] Write clear and concise comments in the code to explain any complex registration logic.

### DAO

- [ ] createUser()

### Service

- [ ] registerUser()

### J-unit Tests

- [ ] testValidRegisterUser()
- [ ] testInvalidRegisterUser()

### Estimation

- [ ] 8 hours

### Flow Chart
```mermaid
graph TD
A[(Start)] --> B([Validate User])
B --> |Valid| C([Check Existing Active User])
B --> |Invalid| E{{Validation Exception}}
C --> |Active User Found| F{{ServiceException}}
C --> |User Not Found| D([Create User])
D --> |Success| I([Registration Successful])
E --> |Fail| G([Registration Failed])
F --> |Fail| G
G --> H[(End)]
I --> H
```

Locate the user DAO and service entry points for the unchecked createUser() and registerUser() tasks, then review the existing JUnit test structure. Add the two registration tests covering valid and invalid users, and confirm the registration flow handles the listed validation and existing-user cases with passing tests.

Written by the indexing model from the issue text.