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/es.ts

@@ -190,6 +190,7 @@ export const es: TranslationResources = {
       notFound: "Agentno encontrado",
       failedToLoad: "No se pudo cargar el agente",
       reconnecting: "Reconectando...",
+      timelineSyncFailed: "No se pudo actualizar el historial del agente. Reintentando…",
       archivingTitle: "Agente de archivo...",
       archivingSubtitle: "Espere mientras archivamos este agente.",
     },