User module | Login User Feature

## User Login Process

### User Story
- As a user, I want to log in with my email and password.
- Correct credentials grant access to my account; incorrect ones display error messages.

### Task: Implement User Login
- [ ] Create Data Access Object (DAO) Layer for user login.
- [ ] Implement Service Layer for user login functionality.
- [ ] Write JUnit Test Cases to ensure the login process works correctly.
- [ ] Verify that all test cases are passing successfully.
- [ ] Add clear and concise comments in the code to explain any complex logic.

### DAO:
- [ ] login()

### Service:
- [ ] loginUser()

### Test:
- [ ] testLoginSuccess()
- [ ] testEmailNotExists()
- [ ] testLoginPasswordCheck()
- [ ] testLoginEmailCheck()

### Estimation
- [ ] 5 hrs

### Flow
``` mermaid
graph TD
A[[Start]] --> B([Validate User])
B --> |Valid| C[(Retrieve User from Database)]
B --> |Invalid| E{{Validation Exception}}
C --> |User Not Found| F{{ServiceException}}
C --> |User Found| D([Check Password Match])
D --> |Returns User| I([Login Successful])
E --> G([Login Failed])
F --> G
D --> |Password Mismatch| G
G --> H[[End]]
I --> H
```