Keep focused agent timelines live and catch up instantly (#2196)

* 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

packages/app/src/screens/workspace/workspace-screen.tsx

@@ -2,6 +2,7 @@ import {
   memo,
   useCallback,
   useEffect,
+  useLayoutEffect,
   useMemo,
   useRef,
   useState,
@@ -2027,7 +2028,7 @@ function WorkspaceScreenContent({
       }),
     [isFocusModeEnabled, isMobile, isRouteFocused, uiTabs, workspaceLayout],
   );
-  useEffect(() => {
+  useLayoutEffect(() => {
     if (!persistenceKey || !viewedTimelineSync) {
       return;
     }