Amazon (AMZN) still has a big lead in the battle for the hearts and wallets of Gen Z shoppers, despite growing investor concern that competition from Temu and Shein could challenge Amazon stock. X Amazon is the favorite shopping website for 59% of American teens, according to a survey by Piper Sandler. The firm reached out to more than 9,000 teenagers for its semiannual Taking Stock With Teens report released Tuesday. Amazon's score marks an increase from 57% this spring and 52% last fall. Behind Amazon, privately held retailer Shein was second, as a favorite of 7% of teens. The…