🧪 Add Playwright tests for batch ZIP generation error handling

[paul0728]

🎯 What: This PR adds a missing automated test to ensure that errors occurring during batch ZIP archive generation are correctly caught and reflected in the UI's failure tally, as defined in docs/index.html:363. It introduces the @playwright/test framework to the repository.

📊 Coverage: The new test suite (tests/zip_generation.spec.js) explicitly targets the batch processing workflow. It uploads dummy images, mocks the JSZip.generateAsync method to reliably throw a simulated error, and verifies the frontend #status div accurately displays the aggregated error count.

✨ Result: Test coverage is significantly improved by automatically verifying one of the most complex frontend workflows (batch processing + zipping). The Playwright testing foundation is also laid, allowing for future end-to-end tests to be added easily. Playwright test reports are properly ignored via .gitignore and npm test works correctly.

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PR created automatically by Jules for task 11917237275967204278 started by @paul0728

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