🧹 [Code Health] Refactor duplicated download link creation logic

[paul0728]

🎯 What: The duplicated code for creating download link anchors has been abstracted out to a central createDownloadLink helper function.
💡 Why: This reduces code duplication in docs/index.html (specifically around single file download links and ZIP batch download links). It ensures that all download links follow a consistent format, style, and structure, improving overall maintainability and reducing the likelihood of future styling inconsistencies.
✅ Verification: Verified via node -c script.js on the extracted inline javascript. Verified functional equivalence via a Playwright UI script, successfully uploading a dummy file and ensuring the download link appears and renders correctly.
✨ Result: Cleaned up code health resulting in better maintainability without changing any functionality.

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PR created automatically by Jules for task 14219608452628774819 started by @paul0728

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