Is a wife permitted to dispose of her own money to purchase non-essential household items or for renovations, even if her husband does not approve, or is that obligatory upon the husband, and she is not allowed to make such purchases unless he is unable to do so?

A wife's participation with her husband in household expenses and affairs is commendable and praiseworthy, and she receives great reward from Allah for it. This is because it strengthens the relationship between the spouses, and it is what the women of the righteous predecessors did, just as Khadijah (may Allah be pleased with her) did with the Prophet (peace and blessings be upon him). The husband should appreciate this from his wife, and the wife should consult her husband concerning matters she wishes to undertake so that she does not do anything of which he disapproves.