Gemini 4 Argon · Lower credit costs · Clearer errors

- Added Gemini 4 Argon for coding, reasoning and multimodal understanding. Select it from the model menu.
- Reduced credit overhead for model calls and improved usage, caching and reserved-credit settlement.
- Model failures now show clearer localized KLB error codes. Retired Luna selections use the current model choices with the correct plan permissions.