GHSA-fmmf-xq98-g327: Vikunja: Write-level project members can delete admin-tier link shares through an unloaded permission check

A project member with Write permission can delete an admin-tier link share on that project. The deletion authorization check reads the permission from an object populated only with URL IDs, rather than from the stored share. Its zero value is Read, so the check falls through to project Write permission instead of requiring Admin.