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/ko.ts

@@ -215,7 +215,8 @@ export const ko: TranslationResources = {
       notFound: "에이전트를 찾을 수 없습니다",
       failedToLoad: "에이전트를 불러오지 못했습니다",
       reconnecting: "다시 연결하는 중",
-      timelineSyncFailed: "에이전트 기록을 새로고침할 수 없습니다. 재시도 중…",
+      timelineSyncFailed: "에이전트 기록을 새로고침할 수 없습니다.",
+      timelineSyncRetrying: "재시도 중…",
       archivingTitle: "에이전트 보관 중...",
       archivingSubtitle: "이 에이전트를 보관하는 동안 잠시 기다려 주세요.",
     },