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.
- [ ] Write clear and concise comments in the code to explain complex logic.

### DAO:

- [x] findUserByEmail()

### Service:

- [x] loginUser()

### Test:

- [x] testLoginSuccess()
- [x] testLoginFailed()

## Flow Chart

```mermaid
graph TD
A[(Start)] --> B([Validate User])
B --> |Valid| C[(Retrieve Email From Database)]
B --> |Invalid| E{{Validation Exception}}
C --> |Email Not Found| F{{ServiceException}}
C --> |Email Found| D([Check Email And Password])
D --> |Fail| G([Login Failed])
D --> |Success| I([Login Successful])
E --> |Fail| G([Login Failed])
F --> |Fail| G
G --> H[(End)]
I --> H

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