fix(navbar): show Dashboard instead of Classes label for admin users

[AbiramiR-27]

Checklist:

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title

Closes #597

This PR dynamically renames the navbar link "Classes" to "Dashboard" for ADMIN users to match the actual destination page.

Changes:

• Added the custom session callback to NextAuth config to expose user roles to the client.
• Added a helper function in Navbar to recursively find and update the label for links to /classes based on user role.
• Added unit tests to navbar.test.jsx to cover both admin and teacher role scenarios.

[NewtonLC]

Hi @AbiramiR-27, thank you for working on this issue! I tested your code, and it works as expected. Before merging this though, I'd like to modify it to also hide this button if the user is not signed in, or if the user has a role other than ADMIN or TEACHER.

Would you be alright with adding those changes? If not, no worries! I can patch it later this week.

[AbiramiR-27]

Thank you for testing the changes and for the feedback! I'd be happy to make the additional updates. I'll modify the implementation to hide the button when the user is not signed in and when the user's role is neither ADMIN nor TEACHER, then update the PR shortly.

[NewtonLC]

Looks good to me! This PR is now awaiting a review from @utsab before merging.