Function probeAuthRequirement

Probe whether the configured server requires authentication, by requesting a normally-protected endpoint (/languages) without a token:

• HTTP 401 -> 'required' (multi-user server: show the login step)
• HTTP 2xx -> 'optional' (single-user server: every endpoint is public)
• anything else / network error -> 'unknown' (caller should assume login)

Must be called after setApiServer but before a token is obtained.