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/ja.ts

@@ -190,6 +190,7 @@ export const ja: TranslationResources = {
       notFound: "エージェントが見つかりません",
       failedToLoad: "エージェントの読み込みに失敗しました",
       reconnecting: "再接続中...",
+      timelineSyncFailed: "エージェントの履歴を更新できませんでした。再試行しています…",
       archivingTitle: "エージェントをアーカイブ中...",
       archivingSubtitle: "このエージェントをアーカイブするまでお待ちください。",
     },