feat: Add OpenAI Compatible API support

[tinylion1024]

• Add multi-provider AI service architecture supporting OpenAI, Groq, Anthropic, local Ollama, and custom endpoints
• Implement unified AI service abstraction layer with automatic provider switching
• Create user-friendly configuration interface with real-time validation
• Add persistent configuration storage in localStorage
• Maintain backward compatibility with existing Portkey integration
• Enhance error handling and fallback mechanisms
• Update documentation and add environment variable examples

New files:

• nextjs-web-app/src/lib/ai-config.ts - AI provider configuration management
• nextjs-web-app/src/lib/ai-service.ts - Unified AI service abstraction
• nextjs-web-app/src/hooks/useAIConfig.ts - Configuration management hook
• nextjs-web-app/src/components/AIConfigModal.tsx - Configuration UI modal
• nextjs-web-app/src/components/AIConfigButton.tsx - Configuration button component
• nextjs-web-app/.env.example - Environment variables template
• nextjs-web-app/src/lib/tests/ai-config.test.ts - Unit tests

Modified files:

• nextjs-web-app/src/app/api/generate/route.ts - Multi-provider API support
• nextjs-web-app/src/app/page.tsx - AI configuration integration
• nextjs-web-app/src/app/results/page.tsx - Provider selection support
• README.md - Updated documentation with new features

Features:

• Support for 6 different AI providers out of the box
• Seamless provider switching without application restart
• Real-time configuration validation with user feedback
• Secure API key storage and management
• Full TypeScript support with proper type safety
• Comprehensive error handling and user-friendly messages
• No breaking changes - fully backward compatible