🧪 [testing improvement description]

[paul0728]

🎯 What: Missing test file for application logic. Added a full E2E testing setup using Playwright.
📊 Coverage: Scenarios covered include Page Load, Format Dropdown initialization, File Selection UI state management, Single Conversion flow, and Batch Conversion flow.
✨ Result: Improved test coverage and reliability of the codebase. The index.html inline JS logic is now continuously verifiable against DOM interactions.

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PR created automatically by Jules for task 17258925174538061357 started by @paul0728

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