assorted playground fixes

[threepointone]

• use streamdown for text
• move the input down
• scroll to the bottom when messages change
• fix the bug where it would stop streaming

[changeset-bot]

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Latest commit: d47dbf4

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[threepointone]

this wasn't used anywhere

[pkg-pr-new]

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commit: d47dbf4

[claude]

Claude Code Review

PR adds streamdown for markdown, fixes stream stop bug, and improves UI layout.

Issues:

1. ai-react.tsx:273 - Code correctly handles empty body and done separately. No issue here on review.

2. app.tsx:195-199 - Auto-scroll useEffect may fire on message updates without new content. Consider checking scroll position or message length.

3. app.tsx:195 - Scroll timing: Streamdown may not finish rendering before scroll. Use requestAnimationFrame.

Testing: No tests for stream stop fix or auto-scroll behavior.

Good: Streamdown integration, sticky input positioning.