feat: Add multi-provider support for OpenRouter and Gemini

[somdipto]

• Implement BaseProvider interface for unified provider management
• Add OpenRouterService with OpenAI-compatible API support
• Enhance existing providers (Anthropic, OpenAI, Google) with BaseProvider interface
• Create ProviderManagerService for dynamic provider switching
• Add API endpoints for provider management (/providers/*)
• Implement provider health checks and model enumeration
• Add comprehensive unit and integration tests
• Update UI for multi-provider API key management
• Add documentation for multi-provider architecture
• Support for provider-specific model selection
• Graceful fallback when providers are unavailable

Fixes #144