⚡ Bolt: memoize chat thread list array operations

[bobdivx]

💡 What: Added useMemo hooks for the rows and current derived states in apps/web/src/components/chat/useThreadList.ts.
🎯 Why: useThreadList processes an array of agents to build nested hierarchies. Because chat polling triggers frequent re-renders, recalculating this array on every render caused unnecessary CPU overhead, especially when the lists become large.
📊 Impact: Reduces unnecessary synchronous array .filter(), .map(), and .sort() operations on every render during polling by memoizing the result until the underlying source data actually changes.
🔬 Measurement: Verify tests still pass using npm run test in apps/web. No visual changes should occur.

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PR created automatically by Jules for task 10345656599905561121 started by @bobdivx

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