fix(app): back off and offer retry when timeline sync is refused

A host can refuse a timeline read for as long as it likes. Codex does
this whenever another writer holds the thread: every catch-up fails with
"already has an active writer" until the other session lets go. The app
retried that on a fixed 1s timer with no ceiling, so an idle window sat
there issuing a request and logging a warning every second, indefinitely.
The refusal is normal lifecycle; the traffic was the bug.

Catch-up and subscription retries now double from 1s to a 30s ceiling,
resetting on success, reconnect, delivery-mode change, and visibility
change so recovery stays immediate once the condition clears.

Backing off alone would leave the user waiting up to 30s with no way in,
so both failure surfaces get a retry they can press. Background retries
stay silent; a retry the user asked for is a fallible user action and
owns its pending state, so "retrying" is a status the sync model
publishes rather than a React boolean.

The blocking "Failed to load agent" screen was the worse half: its load
effect self-gates on the error state, so a refused first resume was
terminal until the user navigated away. Its retry resets the state to
idle, which re-arms the effect.

packages/app/src/i18n/resources/zh-CN.ts

@@ -215,7 +215,8 @@ 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…",
+      timelineSyncFailed: "无法刷新代理历史记录。",
+      timelineSyncRetrying: "正在重试…",
       archivingTitle: "正在归档 Agent...",
       archivingSubtitle: "请稍候，我们正在归档这个 Agent。",
     },