When API Key is selected but unavailable/unconfigured and credits are
available, the card now shows "AI credits in use" with a warning icon
instead of "API key required". When both credits are exhausted and no
API key exists, it shows "No available usage" (destructive).
New deriveVariant logic for priority=apiKey:
- !exhausted + !authorized → credits-fallback (was api-required-*)
- exhausted + no credential → no-usage (was api-required-add)
- exhausted + named unauthorized → api-unavailable (unchanged)