Remove & Retention session only permit "remove backups older than" option value in years.

### Describe the problem, feature or ask a question: I recently updated backtintime to 1.6.1 and I realised that the option "remove backups older than" in session "Remove & Retention" is saving values only in years. It does not accept unit=days for example. If I rollback to 1.5.5 everything goes back to normal. I suppose there is a but in the GUI or something. <img width="556" height="546" alt="Image" src="https://github.com/user-attachments/assets/0d85489e-b829-4887-a551-fba2ba61b7c7" />