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.

docs/timeline-sync.md

@@ -42,6 +42,16 @@ Initialization timeouts guard lack of catch-up progress, not the full multi-page
 Opening or resuming an agent fetches one bounded latest tail page. Older history remains
 user-driven by scrolling upward.
 
+A failed catch-up or subscription reconcile retries on its own, doubling from 1s to a 30s ceiling.
+A fixed 1s retry turned a persistent daemon-side refusal — a Codex thread that already has an active
+writer, say — into a request and a log line every second on an idle app. The delay resets on success,
+reconnect, delivery-mode change, and visibility change, so recovery is still immediate once the
+condition clears.
+
+Background retries are silent. The retry the user presses in the sync-error callout is a fallible
+user action and owns its pending state: `retrying` is a status the sync model publishes, not a React
+boolean, so the button reports in-flight and the callout returns to `error` when the attempt fails.
+
 Reaching the history-start threshold loads one older page and preserves the visible content anchor.
 Cursor progress does not trigger another page. The user must leave and return to the threshold unless
 the anchored page still leaves the viewport at history start, as with short or compacted content; in