fix(web): a thread opened before its shell loads still clears its visit state

The thread structure leaves out lastVisitedAt. Before the thread reaches
the shell index, ChatView builds its shell from that structure, so the
shell had no visit mark and "Woke from snooze" stayed after the visit.
That fallback now takes lastVisitedAt from lastVisitedAtAtom, a live
binding of just the visit mark. With a shell present it is not subscribed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
9d3d988056Julius Marminge committed on 10/22/2026, 5:13:28 AMparentfb94ac6

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