If a husband says to his wife, "It is forbidden for me to see your face again," in a context of anger and without the intention of divorce, is it considered Dhihar or a mere slip of the tongue (laghw)?

A wife's statement "haram upon me" does not entail prohibition (tahrim). If she breaks her oath, she must offer expiation for an oath (kaffarat yameen). As for a husband swearing to prohibit himself from seeing his wife's face, if he does not intend by it divorce or Dhihar, then nothing of that sort occurs. However, he is bound by expiation for an oath (feeding ten poor persons or clothing them, or fasting three days if he cannot find them).