User Module | Login User Feature

# User Login Process

### User Story
As a user, I want to be able to log into the application using my email and password. If my credentials are correct, I should be granted access to my account. If there's an issue with the credentials, I should receive appropriate error messages.

### Task

- [x] Implement Data Access Object (DAO) Layer.
- [x] Implement Service Layer.
- [x] Create JUnit Test Cases.
- [x] Ensure that the tests cases are passing.
- [x] Write clear and concise comments in the code to explain complex logic.

#### DAO
- getUser()

#### Service
- loginUser()

#### Junit Test
- testValidLogin()
- testInvalidLogin()

### Estimation
- 3 hours

# Flowchart

```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

style I fill:#dfd,stroke:#333
style G fill:#fdd,stroke:#333