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

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: #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.

Start at Deck's integration with Nextcloud's BeforeUserDeletedEvent and the existing file-transfer flow described in the issue. Reproduce deletion through Settings → Users, then trace the user-deletion path. Done means owned boards are transferred to the selected account or the admin receives and confirms an explicit board-loss warning.