feat: support ANTHROPIC_API_KEY field and add AiHubMix provider

Add compatibility for providers that use ANTHROPIC_API_KEY instead of
ANTHROPIC_AUTH_TOKEN, enabling support for AiHubMix and similar services.

Changes:
- Backend: Add fallback to ANTHROPIC_API_KEY in credential extraction
  - migration.rs: Support API_KEY during config migration
  - services/provider.rs: Extract credentials from either field
- Frontend: Smart API key field detection and management
  - getApiKeyFromConfig: Read from either field (AUTH_TOKEN priority)
  - setApiKeyInConfig: Write to existing field, preserve user choice
  - hasApiKeyField: Detect presence of either field
- Add AiHubMix provider preset with dual endpoint candidates
- Define apiKeyField metadata for provider presets (documentation)

Technical Details:
- Uses or_else() for zero-cost field fallback in Rust
- Runtime field detection ensures correct field name preservation
- Maintains backward compatibility with existing configurations
- Priority: ANTHROPIC_AUTH_TOKEN > ANTHROPIC_API_KEY

src-tauri/src/services/provider.rs

@@ -1032,6 +1032,7 @@ impl ProviderService {
 
                 let api_key = env
                     .get("ANTHROPIC_AUTH_TOKEN")
+                    .or_else(|| env.get("ANTHROPIC_API_KEY"))
                     .and_then(|v| v.as_str())
                     .ok_or_else(|| {
                         AppError::localized(