feat: mo.ui.file_browser show spinner when listing is slow

[enosh]

📝 Summary

Adds a spinner to mo.ui.file_browser if listing files takes more than 200ms.

🔍 Description of Changes

I was using mo.ui.file_browser with S3, browsing a medium size directory and the initial load takes something like 20 seconds without any indication for what's happening. Felt a bit broken so this changes that.

The delay is there to avoid flashing the spinner when listing is quick.

Here's a demo with the change:

fb-spinner-demo.mov

📋 Checklist

• I have read the contributor guidelines.
• For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
• I have added tests for the changes made.
• I have run the code and verified that it works as expected.

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

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Project	Deployment	Preview	Comments	Updated (UTC)
marimo-docs	Ready	Preview	Comment	Nov 12, 2025 11:28am

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[enosh]

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

[mscolnick]

this is awesome, great implementation!