Syntax modernization: Convert var to let/const for better maintainability

[HendrikCromboom]

🚀 Summary

This PR refactors variable declarations across the codebase by replacing var with let or const to align with modern JavaScript standards. This improves scoping, readability, and prevents potential bugs related to variable hoisting.

✅ Changes Made

• Replaced all instances of var with let or const based on reassignment behavior.
• Applied changes across the following directories:
  • src/
  • tests/
  • modules/
• Verified that all tests pass after refactor.

🧪 How to Test

1. Run the full test suite:

npm test

2. Manually inspect key files for correct scoping and behavior.

3. Confirm no regressions in OAuth flows or provider modules.

📚 Context

This change is part of a broader effort to modernize the codebase and improve maintainability. It also prepares the project for future ES6+ enhancements like tree-shaking and modular imports.

🔗 Related Issues

None

🙌 Acknowledgements

Thanks to the community for highlighting this opportunity for improvement!

📣 Notes for Reviewers

Let me know if you'd prefer this refactor split into smaller PRs by module.

Happy to add JSDoc comments or ESLint rules in a follow-up PR.