Improve Python interpreter detection and exclude virtual environments

The current build configuration has two issues:

1. **Python interpreter detection**: The Makefile relies on the `python` alias, which may not exist on some systems. To ensure compatibility, it should use `python3` when available, while still supporting the python alias if present.

2. **Virtual environment tracking**: The `venv/` directory appears in git status as untracked files, which should not be version controlled.

Solution

Update Makefile to automatically detect python3 with fallback to python.

Add venv/ to .gitignore to exclude virtual environment files.

### Impact

Improves compatibility across different Python environments.

Prevents accidental commit of virtual environment files.