High gas prices and continuing mortgage woes have retailers bracing for a slowdown in consumer spending this holiday season. The latest evidence? Starbucks, which yesterday reported its first-ever quarterly decline in customer visits. Cost-conscious coffee drinkers, it seems, are getting their caffeine high for $1 less per cup at places like McDonald’s and Dunkin Donuts. The stock price of Starbucks is down more than 30 percent this year.