Cookie validation improvements:
1. Validate with activity ID 3350 (more reliable test endpoint)
2. Distinguish 5xx (outage) from 4xx (invalid cookie) during validation
3. On 5xx during validation: preserve cookie, don't re-login (server outage)
4. On 401/403 during validation: clear cookie and re-login
5. On network error during validation: preserve cookie (treat as server issue)
This prevents unnecessary re-logins during server outages.
13eccdd3cc