feat: Add comprehensive Windows compatibility and enhanced LLM model setup

🚀 Major cross-platform enhancement making FSS-Mini-RAG fully Windows and Linux compatible

## Windows Compatibility
- **New Windows installer**: `install_windows.bat` - rock-solid, no-hang installation
- **Simple Windows launcher**: `rag.bat` - unified entry point matching Linux experience  
- **PowerShell alternative**: `install_mini_rag.ps1` for advanced Windows users
- **Cross-platform README**: Side-by-side Linux/Windows commands and examples

## Enhanced LLM Model Setup (Both Platforms)
- **Intelligent model detection**: Automatically detects existing Qwen3 models
- **Interactive model selection**: Choose from qwen3:0.6b, 1.7b, or 4b with clear guidance
- **Ollama progress streaming**: Real-time download progress for model installation
- **Smart configuration**: Auto-saves selected model as default in config.yaml
- **Graceful fallbacks**: Clear guidance when Ollama unavailable

## Installation Experience Improvements
- **Fixed script continuation**: TUI launch no longer terminates installation process
- **Comprehensive model guidance**: Users get proper LLM setup instead of silent failures
- **Complete indexing**: Full codebase indexing (not just code files)
- **Educational flow**: Better explanation of AI features and model choices

## Technical Enhancements
- **Robust error handling**: Installation scripts handle edge cases gracefully
- **Path handling**: Existing cross-platform path utilities work seamlessly on Windows
- **Dependency management**: Clean virtual environment setup on both platforms
- **Configuration persistence**: Model preferences saved for consistent experience

## User Impact
- **Zero-friction Windows adoption**: Windows users get same smooth experience as Linux
- **Complete AI feature setup**: No more "LLM not working" confusion for new users
- **Educational value preserved**: Maintains beginner-friendly approach across platforms
- **Production-ready**: Both platforms now fully functional out-of-the-box

This makes FSS-Mini-RAG truly accessible to the entire developer community! 🎉