NEW YORK — It would have been easy to turn on their computers at home over plates of leftover turkey and take advantage of the Black Friday deals most retailers now offer online. But across the country, thousands of shoppers flocked to stores on Thanksgiving or woke up before dawn the next day to take part in this most famous ritual of American consumerism. Shoppers spent their holiday lined up outside the Mall of America in Bloomington, Minn., by 4 p.m.