Hurricane Irene swept through Manhattan on Sunday but reserved the worst of its fury for towns and suburbs up and down the northeast region where driving rain and flood tides inundated homes and cut power to millions. On its march up the East Coast over the weekend, the storm left at least 15 dead, as many as 3.6 million customers without electricity, widespread flooding and thousands of downed trees. It forced the closure of New York’s mass transit system, and the cancellation of thousands of flights.