Refactor content_script.js: improve readability, structure, and best practices

[galanggg]

This PR refactors content_script.js to apply JavaScript best practices:

• Extracts button creation into a helper function
• Uses modern JavaScript (const/let, optional chaining, arrow functions)
• Improves event handling readability
• Suggests moving inline styles to CSS for maintainability
• Adds better structure and inline documentation
• Avoids global variable leaks

No logic is changed; behavior remains identical. This improves maintainability and prepares for future enhancements.

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File changes:

• Refactored content_script.js as per best practices (see file diff for details)

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