Is it necessary to perform ghusl after masturbating at night, and what is the ruling on delaying it until morning? Is washing the hair a condition, and does Allah forgive the sin of someone who resisted it but committed it involuntarily?

The questioner must repent to Allah for this habit. Ghusl (ritual bathing) is not obligatory merely by performing the habit; rather, it becomes obligatory if semen is discharged. If ghusl is required, the hair must be washed along with the rest of the body, and ghusl is not valid without it.