🔧 Revamp UI Elements for Enhanced Usability

[mlamina]

This pull request updates several UI components to improve usability and design consistency across the application.

🎨 UI Enhancements

• Updated button styles in templates/base.html to use outlined dark theme for better contrast.
• Modified navbar items in templates/base.html for a more cohesive look.

🖼️ Image Assets

• Replaced favicon and Open Graph image with orb.png in static/img/orb.png.

📝 Template Adjustments

• Enhanced form layout in pr_manager/templates/description_generator_form.html for better user interaction.
• Improved tab and dropdown styles in pr_manager/templates/view_pull_request.html for a streamlined user experience.

These changes aim to provide a more intuitive and visually appealing interface for users, enhancing overall interaction with the application.