Implement authentication and user management:

- Add JWT-based authentication with `JwtUtils` and `JwtAuthenticationFilter`.
- Create `AuthController` for login and registration endpoints.
- Add `UserController` for managing users with admin restrictions.
- Introduce backend configurations: `ApplicationConfig` and `SecurityConfig` for Spring Security setup.
- Develop new domain model `User` and repository `UserRepository`.
- Implement database initialization logic for default admin user creation.
- Extend frontend with `Login` and `UserManagement` components.
- Enhance `authService` with token storage, authentication checks, and role validation.
- Add JWT-based user roles for permission handling in both backend and frontend.