🧪 Add tests for loadFormats function

[paul0728]

🎯 What: The testing gap addressed
This PR addresses the missing tests for the loadFormats function inside docs/index.html. Since the application logic resides mostly in inline module scripts, testing required setting up jest and jsdom, manually instantiating a DOM, mocking global implementations of fetch, and handling ESM script evaluation correctly within the JSDOM context.

📊 Coverage: What scenarios are now tested

• Happy Path: Ensures the function successfully fetches format_database.json, parses it, and correctly populates the DOM elements (restricting the accept attribute of fileInput and adding the write-supported formats to the targetFormat dropdown options, e.g., excluding heic).
• Network Error: Ensures the function gracefully falls back to basic formats (png, jpg, webp, pdf) when the fetch request encounters a network error (simulated via response.ok = false).
• Malformed JSON: Ensures the function gracefully handles invalid JSON responses as a fallback, similarly populating the default basic formats.

✨ Result: The improvement in test coverage
The frontend's initial bootstrap flow is now verified, ensuring robustness against backend/network/JSON failures and that valid formats properly propagate from the database file to the web UI options.

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PR created automatically by Jules for task 6577386116842439749 started by @paul0728

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