Is it permissible to cut one's hair before performing ghusl for janabah, and what is the evidence for that?

There is no harm for a person in a state of major ritual impurity (junub) to remove their hair or nails, as the default is permissibility, and no sound evidence prohibits it. It has been reported that the Prophet, peace and blessings be upon him, said: "Indeed, a believer does not become impure (najis)." No legal evidence has been reported that disapproves of removing hair or nails for a person in a state of major ritual impurity. The Prophet, peace and blessings be upon him, commanded a person who embraced Islam to bathe, remove the hair of disbelief, and undergo circumcision. He did not command him to delay circumcision or hair removal until after bathing. Similarly, a menstruating woman is commanded to comb her hair when she washes, even though combing removes some hair.