When the server returns HTTP 500 or other 5xx errors, the client
was caching the error HTML page (e.g., Nginx error pages) and
attempting to parse it as JSON on boot, causing a lockout with
the message 'Could not process config and no cached copy'.
This fix adds all standard 5xx server error codes to the
offlineStatusCodes list, ensuring the client enters offline mode
and uses cached data instead of caching error responses.
Before: Only 502, 503, 504, 530 triggered offline mode
After: All 5xx errors (500-511, 530) trigger offline mode
Fixes#1724