`sharing.folders` is present in `DEFAULT_USER_PERMISSIONS` but absent from the `SharingPermissions` response/update schema, so the admin default and group permission API silently drops it on every round-trip and the setting is never saved. Add `folders: bool = False`, matching the config default (`USER_PERMISSIONS_FOLDERS_ALLOW_SHARING`), restoring parity with `DEFAULT_USER_PERMISSIONS`.
Fixes#27120.