fix: Prevent cleanup messages when showing help or usage

- Add SHOW_HELP and LIST_PARTITIONS_ONLY flags to track display modes
- Modified cleanup() function to skip cleanup messages for help/usage scenarios
- Set appropriate flags in usage() function and error conditions
- Cleanup messages now only appear during actual WIC image processing

Fixes cases where 'Cleaning up...' appeared when running:
- ./wic-editor.sh (no arguments)
- ./wic-editor.sh -h (help)
- ./wic-editor.sh -i image.wic -p list (partition listing)
- Error conditions that show usage

Improves user experience by keeping help output clean and focused.

Author: Alex J Lennon