The student pilot was performing a solo cross country flight and was on final approach at the destination airport. The pilot stated that the airplane speed decreased while crossing the runway threshold, which resulted in an aerodynamic stall and the airplane landing hard on the main landing gear. The airplane subsequently porpoised, and the nose wheel landing gear collapsed. The airplane sustained substantial damage to the firewall. The pilot reported no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.