Real estate agents nationwide are working seven days a week, builders are staying open late and homebuyers are scrambling to get their offers in as they rush to take advantage of tax credits that expire at midnight today. To qualify, buyers must have a signed contract in hand by the deadline and must complete the deal by June 30. The tax incentives — offered to both first-time buyers and some current homeowners — are fueling a strong spring selling season and helping home prices stabilize. Real estate agents hope the burst in activity, along with the lifting of general economic gloom, will propel the housing market for the rest of the year.