Fix React Error 185 in ChatView by deferring markThreadVisited

[gleber]

Summary

Fixes React error pingdotgg#185 ("Maximum update depth exceeded") in React 19 caused by synchronous state updates within a single React commit cycle.

Details

In apps/web/src/components/ChatView.tsx, the useEffect that calls markThreadVisited(serverThread.id) was triggering a synchronous Zustand store update. This caused a React re-render, which re-ran the effect and potentially created an infinite update loop.

This PR wraps the markThreadVisited call in startTransition, which tells React to treat the update as non-urgent. This defers the state update out of the synchronous commit cycle, breaking the loop and avoiding the error.

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PR created automatically by Jules for task 11849620451264782131 started by @gleber

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