Add PyInstaller build system for single executable distribution

- Added requirements-build.txt with PyInstaller dependencies
- Created fixture-manager.spec with comprehensive build configuration
- Added build.py automated build script with full workflow
- Created BUILD.md with detailed build instructions and troubleshooting
- Updated README.md with build section and executable distribution info
- Includes hooks for Flask-SQLAlchemy and pandas compatibility
- Supports cross-platform builds (Linux, Windows, macOS)
- Creates distribution package with executable and essential files
- Executable size ~80-120MB with all dependencies included
- No Python installation required on target systems