Update build script to check dependencies instead of auto-installing

✅ SAFER BUILD PROCESS:
- Changed install_dependencies() to check_dependencies()
- No longer automatically installs PyQt6/pyinstaller
- Provides clear instructions if dependencies are missing
- User must manually install: pip install -r requirements.txt
- Added --check-deps option for dependency verification only

TECHNICAL DETAILS:
- More secure: doesn't modify user's Python environment
- Better control: user chooses when to install packages
- Clear feedback: shows which packages are missing
- Improved workflow: separates dependency management from building

USER WORKFLOW:
1. pip install -r requirements.txt (manual)
2. python3 build_usb_creator.py (checks deps then builds)
3. Executable created if all dependencies satisfied