fix(web): mark newly discovered completed threads unread

[danieliser]

Supersedes #4501, which GitHub cannot reopen after its head branch was force-pushed.

Rebased onto current main (2026-08-23).

This preserves the focused behavior: after a snapshot environment finishes bootstrap, a thread first discovered already completed is marked unread; initial history remains read.

Note

Mark newly discovered completed threads as unread in EventRouter

• Adds environmentSnapshotIdsAtom to derive the set of environment IDs that currently have a snapshot present
• Adds resolveFirstSeenCompletedThreads, a pure function that compares current threads against previously seen thread keys and returns completed threads that are newly visible, plus the next seen-key snapshot
• Adds useMarkFirstSeenCompletedThreadsUnread hook in __root.tsx which runs resolveFirstSeenCompletedThreads on thread/snapshot changes and calls useUiStateStore.getState().markThreadUnread for each new completed thread
• On first run per environment the hook seeds seen keys without marking anything unread; running threads and threads outside snapshot environments are ignored
• Behavioral Change: completed threads that first appear after bootstrap within snapshot environments will now show as unread in the UI

^{Macroscope summarized 9e60709.}

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