- Extended ProviderConfig with optional custom fields for non-OpenAI APIs
- Added custom_endpoint_path, custom_auth_header, custom_auth_prefix fields
- Added api_format field to distinguish between OpenAI and Custom REST provider formats
- Added session_id field for stateful conversation APIs
- Implemented chat_custom_rest() method in OpenAI provider
- Custom REST provider uses different request format (prompt+sessionId) and response (msg field)
- Updated TypeScript types to match Rust schema
- Added UI controls in Settings/AIProviders for custom provider configuration
- API format selector auto-populates appropriate defaults (OpenAI vs Custom REST provider)
- Backward compatible: existing providers default to OpenAI format
