feat: Add Windows PowerShell compatibility
- Updated config.go to use platform-specific paths (AppData on Windows, .config on Linux/macOS)
- Added build.ps1 PowerShell script for building on Windows
- Updated Makefile with cross-platform build targets (build-windows, build-linux)
- Enhanced README.md with Windows-specific installation and usage instructions
- Added WINDOWS.md with detailed Windows compatibility documentation
- Verified go-keyring integration with Windows Credential Manager (via wincred)
The application now fully supports Windows PowerShell while maintaining backward compatibility with Linux and macOS.