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/i18n/resources/ru.ts

@@ -190,6 +190,7 @@ export const ru: TranslationResources = {
       notFound: "Agent не найден",
       failedToLoad: "Не удалось загрузить агент",
       reconnecting: "Повторное подключение...",
+      timelineSyncFailed: "Не удалось обновить историю агента. Повторная попытка…",
       archivingTitle: "Архивный агент...",
       archivingSubtitle: "Пожалуйста, подождите, пока мы архивируем этого агента.",
     },