Enhance Authentication Command with Modular Design and Improved Error Handling

[Harshit457]

This pull request refactors the auth command to improve modularity, error handling, and user experience.
Key Changes:

1. Extracted authentication logic into a dedicated authenticate() function for better separation of concerns and reusability.
2. Enhanced error handling:
   . Added validation for empty API tokens.
   . Provided user-friendly error messages and feedback.
   . Used %w in fmt.Errorf for error wrapping.
3. Improved code readability and maintainability:
   . Clear separation between command registration (authCmd) and authentication logic (authenticate).
   . Added meaningful Long description to the command.
4. Ensured proper formatting of user prompts and input feedback.