fix: honor Gradio output directory setting

[200420052024xw-dot]

Summary

• add an explicit save button for the Gradio output directory
• create and validate the selected directory before applying it
• persist the confirmed directory in browser state
• pass the configured output root through the conversion pipeline
• show localized success and error notifications

Tests

• added focused output-directory unit tests
• 9 passed
• verified Python syntax and focused Ruff checks
• verified the Gradio 6.8 interface can be constructed successfully

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[200420052024xw-dot]

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[200420052024xw-dot]

I have read the CLA Document and I hereby sign the CLA