What is the ruling on a wife saying to her husband: "You are forbidden to me until the Day of Judgment"? And what should she do to avoid swearing, and will she be held accountable for that?

If the wife says to her husband: "You are forbidden to me," or "You are forbidden to me until the Day of Judgment," this does not constitute ẓihār (a form of divorce) or divorce. Rather, it falls under the category of forbidding what is permissible. She is required to offer an expiation for a broken oath (kaffārat yamīn) if she goes back on her word, which is freeing a slave, or feeding ten needy persons, or clothing them. If one cannot find these, then one must fast for three days. Such a prohibition is not permissible Islamically, and she must repent to Allah.