Hard delete items in decrypt box

I think it would be preferable to hard delete text that is pasted into the DECRYPT box, rather than allow edit undo, once it is deleted.

It is currently possible to EDIT -> UNDO (CMD + Z) to restore text pasted into the DECRYPT box, after it is deleted, which could potentially compromise security.

The issue names no file, test, or entry point; begin by locating the DECRYPT box implementation and its undo handling. Verify the behavior around deleting pasted text, then confirm that the deleted text cannot be restored through Edit → Undo.

Written by the indexing model from the issue text.