Invoice Import: Allow User to Change Unit

<img width="872" height="53" alt="Image" src="https://github.com/user-attachments/assets/2eeb04f2-7980-4abf-b540-7165bc285a06" />

In case the unit has been incorrectly recognized or not clearly specified in the original invoice, the user should be able to change it to one of the predefined units via  a dropdown field.

