Fixing main menu to be fixed

[ger619]

This pull request includes several updates to the app/views/layouts/_navbar.html.erb file to enhance the navigation bar's design and functionality. The key changes involve modifying the header styling, adjusting the hamburger menu for mobile, and adding a scroll event listener to change the header's position.

Navbar Design and Functionality Enhancements:

• Updated the header's class and added an ID to improve styling and positioning, including making it fixed on scroll.
• Adjusted the styling of the hamburger menu button for mobile devices, including increasing the icon size.
• Added a closing div tag to ensure proper HTML structure.
• Implemented a scroll event listener to add a fixed-header class when the user scrolls down, making the header fixed at the top.