How can one reconcile the belief that divine preordainment (qadar) entails both good and bad, with the hadith: "Wondrous is the affair of the believer, for all his affairs are good," which indicates that a believer's preordainment contains no evil?

The reconciliation between belief in divine decree (Qadar), both its good and its evil, and the idea that all affairs of a believer are good, is straightforward.

Belief in divine decree is one of the pillars of faith, meaning the affirmation that everything that occurs in the universe is by Allah’s decree and creation. All of Allah’s decrees are good due to His ultimate wisdom, and what appears as evil to a servant may actually be good for him, such as expiation for sins.

The Hadith "Wondrous is the affair of the believer" means that all the affairs of a believer are good for him in terms of reward and closeness to Allah. In prosperity, he gives thanks, and in adversity, he perseveres, thus earning reward and recompense in both situations.