bugfix-delete-nonexisting-space-link-returns-404.md

Bugfix: Return 404 when removing a non-existing public or user share

RemovePublicShare and RemoveShare returned CODE_INTERNAL when the share to be deleted was not found, causing callers to receive a 500 error instead of the correct 404. Both handlers now propagate IsNotFound errors from the share manager as CODE_NOT_FOUND.

#581