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/fr.ts

@@ -191,6 +191,7 @@ export const fr: TranslationResources = {
       notFound: "Agentintrouvable",
       failedToLoad: "Échec du chargement de l'agent",
       reconnecting: "Reconnexion...",
+      timelineSyncFailed: "Impossible d’actualiser l’historique de l’agent. Nouvelle tentative…",
       archivingTitle: "Agent d'archivage...",
       archivingSubtitle: "Veuillez patienter pendant que nous archivons cet agent.",
     },