automatically suspend synchronization of a disappeared group folder: avoid deleting currently synced files

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### Feature description
this proposed solution deleted all synced data of the group folders - some 150GB - in my case
https://github.com/nextcloud/fulltextsearch/issues/858#issuecomment-2294889748

I suggest to check the size of the deleted files and propose to ask "Do you really want to delete xxxGB"

It's just the reverse situation of checking/asking if the client should download large directories.