Starbucks Corp. is donating a portion of its proceeds on World AIDS Day, Saturday to help fight the disease.   The company announced Thursday it would give five cents to the Global Fund of the RED foundation for each hand-crafted drink it sells on Saturday in the United States and Canada. The coffee shop is also partnering with iTunes, offering a digital $30 gift card that is divided into two $15 certificates to iTunes and Starbucks. Five percent of the proceeds to those gift cards, sold online, will also be donated to RED.   RED was founded by Bono and Bobby Shriver and links with brands to help raise money to fight AIDS. It benefits the Global Fund, which uses the money for HIV/AIDS programs in Africa.