Starbucks has unveiled a new iPhone app that will allow customers to tip from their smartphone, a move that flexes the coffee giant’s digital muscle and could help line baristas’ pockets with money from the millions who use mobile devices to pay for their lattes. Right now all tipping at Starbucks is done in cash. Customers paying with a credit or debit card or with the Starbucks card cannot leave tips unless they have cash in hand.