Use latest user message time for thread timestamps - #1996

Eligibility
This PR combines a minor timestamp display change with a substantial refactor of thread state management in store.ts, including removal of syncServerReadModel and ~300 lines of related tests. An unresolved high-severity review comment identifies that the refactor may cause newly created threads to not appear in the sidebar until a separate shell event arrives, directly impacting the feature this PR aims to implement.

Notes:
No code owners found in target merge base 1bf048e