feat: add PDF export action in upload flow

[hhhjin]

Summary

• add a new Create PDF action to the upload form
• generate printable HTML from parsed markdown using the shared template
• trigger browser print in a hidden iframe with loading/disabled states
• add i18n strings for the new PDF action in en/ko/zh/ja

Testing

• pnpm lint
• pnpm test -- --runInBand

[gemini-code-assist]

Code Review

This pull request introduces a PDF generation and printing feature, adding a 'Create PDF' button to the upload interface along with corresponding translations and state management. The implementation uses a hidden iframe to trigger the browser's print dialog for the rendered markdown. Feedback focuses on enhancing the robustness of the printing utility by addressing a potential memory leak during iframe cleanup and replacing fixed timeouts with callbacks to more accurately manage the generation state.