v0.9.3: Simplified API key handling
- Unified API key resolution: all providers use pure env-var-based design
- MiMo / Xiaomi: display_name and api_key_env renamed, legacy fallback removed
- Removed legacy MIMO_API_KEY fallback from code and documentation
- Provider API key flow: config.json defines env var name → Settings writes via setx → proxy reads via std::env::var