Refactor popup.js for maintainability and modular structure

Problem

The popup.js file contains duplicate function definitions (addAuthor, updateAuthorNumbers) both at the top-level and inside the $(document).ready() block. This makes the code harder to read and maintain.

Suggested Changes

• Remove the top-level function definitions.
• Organize all functionality inside $(document).ready() using a modular structure.
• Optionally use IIFE or a namespace object to encapsulate functions and state.
• Create a new function showStatusMessage() to replace alert-based feedback with UI messages.

Benefit

Improves maintainability and sets the stage for better testing and modular development.