fix: show completed status for first-seen threads

[StiensWout]

Summary

• Mark first-seen completed shell threads unread after an environment shell snapshot exists.
• Reuse the existing markThreadUnread helper so the sidebar can surface the unread Completed status.
• Replaces fix: show completed status for first-seen threads #3139 with a fresh branch based on current main.

Root cause

Newly observed shell threads seed threadLastVisitedAtById from thread.updatedAt ?? thread.createdAt. When a completed thread first arrives through thread-upserted, that seeded timestamp can equal or be later than latestTurn.completedAt, so the sidebar strict comparison treats the completed turn as already seen.

Impact

Background threads that finish before the web client observes them now show the unread Completed status instead of silently appearing idle/read. Initial snapshot bootstrap behavior is unchanged because this only runs after a real per-environment shell snapshot exists.

Validation

• PATH="$HOME/.vite-plus/bin:$PATH" vp test apps/web/src/uiStateStore.test.ts apps/web/src/components/Sidebar.logic.test.ts
• PATH="$HOME/.vite-plus/bin:$PATH" vp check
• PATH="$HOME/.vite-plus/bin:$PATH" vp run typecheck

vp check reports the repository's existing unrelated warnings and no errors.

Closes #3131

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Note

Low Risk
UI-only unread/visit bookkeeping with a narrow trigger (first sighting + completed turn); no auth or server changes.

Overview
Fixes background threads that finish before the client sees them showing as read instead of Completed unread in the sidebar.

Adds useMarkFirstSeenCompletedThreadsUnread, which runs only after each environment has a real shell snapshot (not during initial bootstrap). It tracks which thread keys were already observed per environment; when a thread appears for the first time with latestTurn.state === "completed", it calls the existing markThreadUnread with completedAt so visit timestamps no longer treat the turn as already seen.

The hook is mounted from EventRouter in the authenticated app shell alongside other global event routing.

^{Reviewed by Cursor Bugbot for commit c87567a. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Mark first-seen completed threads as unread in snapshot environments

• Adds useMarkFirstSeenCompletedThreadsUnread, a hook that tracks which threads have been seen per environment and marks newly seen completed threads as unread using their completedAt timestamp.
• Adds environmentSnapshotIdsAtom to derive the list of environment IDs that currently have a snapshot, used to scope the hook's behavior.
• Mounts the hook in EventRouter so it runs globally during the app lifecycle.

^{Macroscope summarized c87567a.}

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR introduces new user-facing behavior by automatically marking first-seen completed threads as unread. While the implementation is clean and self-contained, new features affecting notification/read state warrant human review.

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