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/zh-CN.ts

@@ -190,6 +190,7 @@ export const zhCN: TranslationResources = {
       notFound: "未找到 Agent",
       failedToLoad: "加载 Agent 失败",
       reconnecting: "正在重连...",
+      timelineSyncFailed: "无法刷新代理历史记录。正在重试…",
       archivingTitle: "正在归档 Agent...",
       archivingSubtitle: "请稍候，我们正在归档这个 Agent。",
     },