Implement Login Screen

[a01wielo]

Description:
Als Kunde möchte ich, dass das Verwaltungssystem eine Login Maske integriert, um die User zu identifizieren.

Acceptance Criteria:

• The login screen should include fields for username and password.

• There should be a "Login" button that triggers the authentication process.

• Implement validation for both fields (e.g., non-empty, valid email format for username).

• Display an error message for invalid login attempts.

Tasks:

• Design the login screen layout.
• Implement the login form with necessary fields.
• Add client-side validation for the form.
• Integrate with the backend for authentication.
• Handle error messages and display them to the user.