Add back the missing direct download link button that was removed.
This button allows users to copy a direct download URL for shared files,
which is different from the share page URL.
Changes:
- Add direct download link button with content_paste_go icon
- Import pub API for download URL generation
- Add hasDownloadLink() method to check if file can be directly downloaded
- Add buildDownloadLink() method to generate direct download URLs
- Only show button for single file selections (not directories)
🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>
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