Assistant: restore shows an error on the benign 404, and drops the session id on 5xx

[dannon]

The restore effect in app/hooks/useAssistantChat.ts:97-101 catches every failure the same way:

.catch(() => {
  if (cancelled) return;
  if (!initialSessionId) localStorage.removeItem(SESSION_KEY);
  setError("Failed to restore the previous conversation.");
})

Two problems, and they are inverses of each other:

• 404 is benign. The session expired (2h TTL) or was flushed (Assistant: startup cache flush wipes all live sessions #1523). Nothing is broken -- the right behavior is to clear the stale localStorage pointer and silently start a fresh conversation. Instead the user gets an error banner on a page they just loaded.
• 5xx / timeout is the real failure, and it is the one case where we should NOT clear localStorage. The session may still be alive server-side; dropping the pointer turns a transient blip into permanent loss of the conversation for that user.

So the fix is to branch on status: 404 -> remove the key, no error banner. Anything else -> keep the key, show a retryable error.

This came up in the beta-readiness review as the "session-restore data loss" item. The startup flush in #1523 makes it fire considerably more often than that review assumed.