When a selected subfolder had the exact same name as its parent volume
mount, the `path.relative` check falsely identified the subfolder path
as an absolute path already inside the volume. This caused the entire
volume root to be backed up instead of the intended subfolder.
Closes#250
<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit
* **Bug Fixes**
* Improved handling of backup patterns when folder names match volume names.
* Enhanced error handling for snapshot deletion operations.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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