sendKeepalive() turned a failed fetch, a timeout or a gateway's
502/503/504 into a failed outcome without telling the status, so the
instance stayed verified: when the hidden tab came back, its refresh
skipped the /instance check and resent the queued writes, which could
reach a replacement database behind the same address. Like requests, a
keepalive write now reports these as offline, so the instance is
checked before anything goes out again.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>