* fix(app): keep focused agent timelines live
Window focus was conflated with app visibility, so switching OS windows could remove the selected timeline subscription. Separate those signals, grace every visibility-driven removal, and cover retained history until authoritative catch-up completes.
* fix(app): keep timeline catch-up failures non-blocking
* fix(app): surface timeline catch-up failures
* fix(app): preserve file preview refocus
* fix(app): preserve optimistic catch-up flow
