perf(web): avoid sidebar visit churn during streaming

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What Changed

• Mark the active thread visited when its latest turn completes instead of on every updatedAt stream chunk.
• Move the necessary external-store synchronization into the named useMarkActiveThreadVisited hook.

Why

React Doctor and a manual hot-spot pass identified ChatViewContent/sidebar state fan-out as a high-frequency path. Each streamed chunk previously wrote a new visit timestamp to Zustand, invalidating SidebarProjectGroup's visit selector. The unread indicator compares against latestTurn.completedAt, so recording that timestamp once preserves read/unread behavior while removing intermediate writes.

UI Changes

React Scan was enabled for the same simulated 20-chunk turn in both recordings.

Before: SidebarProjectGroup rendered 20 extra times (2 → 22), with a React Scan highlight on every chunk.

react_scan_thread_visit_before.mp4

After: it stayed stable through streaming and rendered once at completion (1 → 2).

react_scan_thread_visit_after.mp4

Validation

• vp check
• vp run typecheck
• 71 focused uiStateStore and Sidebar.logic tests
• React Doctor performance scan (remaining findings are pre-existing and outside this focused fix)

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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Note

Avoid sidebar visit churn during streaming by extracting useMarkActiveThreadVisited hook

• Extracts visitation-tracking logic from ChatViewContent into a new useMarkActiveThreadVisited hook.
• The hook marks a thread visited whenever latestTurnCompletedAt changes and is non-null, replacing the previous logic that compared serverThread.updatedAt against a stored last-visited timestamp.
• Behavioral Change: visited marking no longer fires during streaming (when completedAt is null), reducing spurious store updates mid-stream.

^{Macroscope summarized 0381ec7.}

[t3-code]

closing as superseded by #5486. that merged change removes automatic visit-on-view/update behavior, so the streaming-time visit writes this pr optimizes no longer exist. thanks for the work here.