Fixes#588.
Sign-out already cleared the token-refresh timers and stopped the
keep-alive interval - the reported endless loops came from async
callbacks that were in flight at that moment. The token refresh's
failure handler re-armed its retry after logout, and a failing
keep-alive ping called reconnect() -> connect(), which restarts the
keep-alive and thereby revived the interval disconnect() had just
stopped. Only closing the tab ended it.
Two mechanisms fix that class: transiently failed token refreshes only
re-arm while the account is still signed in (checked when the failure
lands, not when the request started), and the client carries an
intentionallyDisconnected flag set by disconnect() - the ping callback,
reconnect(), the SSE reconnect scheduling and the polling fallback all
stop at it, so nothing revives after an intentional sign-out.
Failed retries also back off instead of hammering a down server every
30 seconds: the token refresh climbs 30s/1m/2m/5m (capped, reset on
success), and the keep-alive skips upcoming ticks on consecutive
failures for the same effective ladder. Recovery after an outage is
unchanged in substance - the session survives and reconnects within at
most ~5 minutes, immediately on user activity.
