[feature] Redesign Sign-up, Login Pages and navbar in Agent Arena

[Bluesparx]

Problem
The current Agent Arena project lacks modern UI elements and user-friendly design. This results in a less engaging experience for new users. These entry points are essential for a good first impression.

Solution
Redesigning the sign-up, login pages and navbar to enhance the user experience. Specifically:

• Introduce a modern and responsive design with visual improvements, such as better form layout and user feedback.
• Ensure that the pages provide clear error handling and success feedback to users.
• Make the design consistent with the broader branding of the project.

Additional context

Please let me know if I can work on this.