Update dependencies and docs: PyQt5 → PySide6 migration

- Update README.md: replace PyQt5 badge and system requirements with PySide6
- PythonBox_v8.py: code already migrated to PySide6
- requirements.txt: already uses PySide6
- PySide6 uses LGPL license (more permissive than PyQt's GPL)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Lukas Geiger