This avoids distracting the user during active chat responses.

https://claude.ai/code/session_018bxH7uX9t4TH4wxh9VE7Ka

#skip-bugbot

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## Summary by cubic
Hide the uncommitted files banner while chat is streaming to reduce
distraction. Adds an isStreaming check in ChatHeader so the banner only
appears when not streaming.

<sup>Written for commit 4bd7b9fe48f94414e83ae9e4600b4cf898535c1b.
Summary will update on new commits.</sup>

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Co-authored-by: Claude <noreply@anthropic.com>