Boards are silently deleted when their owner is removed — no warning, no transfer option

[ramteid]

Describe the bug

When a Nextcloud admin deletes a user, the admin UI offers to transfer that user's files to another account. However, Deck boards owned by the deleted user are silently deleted without warning and without any transfer option. The admin is never notified that boards will be lost.

Steps to reproduce

1. Create one or more Deck boards as (or shared with) a user who owns them.
2. As an admin, navigate to Settings → Users and delete that user.
3. In the deletion dialog, choose to transfer files to another user and confirm.
4. Observe: the files are transferred correctly, but all Deck boards owned by the deleted user are gone — no warning was shown, and no transfer option was offered for boards.

Expected behavior

Deck should integrate with Nextcloud's user-deletion hook (OCP\User\Events\BeforeUserDeletedEvent / file-transfer mechanism) so that at least one of the following happens:

• Board ownership is transferred to the user selected in the "transfer files to" dialog, or
• The admin is clearly warned that boards will be permanently deleted and must confirm this explicitly before the deletion proceeds.

Silently discarding boards during user deletion is a data-loss scenario. Users have no way to recover the content once the account is removed.

Additional context

• Related: Deleting a Board of a deleted user not possible #1163 — admins can sometimes end up with orphaned boards they cannot manage; the root cause is the same missing integration with the user-deletion lifecycle.
• Nextcloud's core already provides a clean hook for apps to participate in user deletion and the file-transfer flow; Deck should register a handler there.

[luka-nextcloud]

I would prefer the solution described in #5090

[ramteid]

I would prefer the solution described in #5090

Yes that would work, too. It's like in WhatsApp groups if the only admin leaves - an arbitrary user gets admin rights.

However, I believe that this is a suboptimal solution for a software that is used professionally in business cases. It could give the wrong user admin rights, going potentially unnoticed by actual admins.

Also, it would be counter-intuitive for the user (admin) that performs the user deletion: Nextcloud already offers to define another user that receives all files of the deleted user. Why does this not work the same way for their Deck boards? This is a major design flaw in my opinion.