Is your feature request related to a problem? Please describe.
Completed dates are currently shown in the details footer tooltip as read-only data. Sometimes I mark tasks completed only a few days after having actually completed them, but I would like to be able to edit the completed date to reflect when it actually happened.

Describe the solution you'd like
It would be nice to be able to edit completed dates directly into Nextcloud without having to use another piece of software only for this task. It could be implemented as proposed in the issue #312. The field should probably only be shown if a task has been marked as completed. Here is the mockup originally proposed by @TruputiGalvotas:

Describe alternatives you've considered
I am currently using Thunderbird to do this, but I'd rather stick to Nextcloud.