The flight crew reported that during a civilian positioning flight to pick up a medical patient, they were cleared to land via a visual approach at night. During the base segment in the traffic pattern, the first officer, who was the pilot flying, lost sight of the runway and transferred the flight controls to the captain at an altitude of about 900 ft mean sea level. The first officer stated that after the exchange of the flight controls, the “aircraft was on short final and passed over the threshold at an angle.” Subsequently, during the captain’s landing flare and touchdown, the stick shaker activated, and the left wing dropped and impacted the runway. The captain completed the landing roll, stopped the airplane on the runway, and all of the occupants exited without incident. The impact with the runway resulted in substantial damage to the left wing. The flight crew did not report any preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.