⚡ Bolt: Throttle chat scroll listener to reduce reflows

[nkissick-del]

💡 What: Throttled the scroll event listener in useScrollToBottom hook using lodash.throttle (200ms).

🎯 Why: The scroll event fires very frequently. The handler checks checkIfUserAtBottom which reads layout properties (scrollTop, scrollHeight), forcing a reflow (layout calculation) on every event. This causes performance issues (jank) during streaming or manual scrolling.

📊 Impact: Reduces main thread blocking and layout thrashing during scrolling.

🔬 Measurement: Verified using tsc that the code is type-safe. Manual verification of logic correctness (memoized handler, proper cleanup).

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PR created automatically by Jules for task 9272284254319496958 started by @nkissick-del

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Summary by cubic

Throttled the chat scroll listener in useScrollToBottom to 200ms with lodash.throttle to reduce layout reflows and jank during scrolling and streaming. Memoized the handler and cancel it on cleanup to avoid extra work.

^{Written for commit dffa3d0. Summary will update on new commits.}

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[cubic-dev-ai]

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