Update macos-build-script.sh

Improve build script logging and structure

- Restructure build script into numbered steps for better clarity.
- Add logging messages before major steps using 'echo' to print status.
- Use 'INFO', 'ERROR', and 'STEP' prefixes for log readout.
- Validate required data files exist before continuing build.
- Handle Python environment setup and failures more robustly.
- Download required models, libs and dependencies within build script.
- Package app using pyinstaller spec file for distribution.
- Copy built app bundle to Downloads folder for easy install.
- Script has passed test on M1 Max Macbook Pro.