perf(timeline): cache derived timeline data

[tyulyukov]

Summary

• Cache derived work log, timeline row, and timeline entry computations to avoid recomputing the same projections on unchanged inputs.
• Scope timeline derivation to the latest turn where applicable, reducing unnecessary iteration over unrelated activities.
• Stabilize MessagesTimeline remount and scroll state when switching threads without forcing a full component reset.
• Update chat view selectors to read thread shells and rate-limit activity data more directly across environments.

Testing

• Not run (PR description only).
• bun fmt
• bun lint
• bun typecheck
• Relevant bun run test suites for timeline/session derivation behavior