Home Depot, the nation's largest do-it-yourself chain, is gearing up for a major push into high-end installed sales. After testing the concept with seven stores in five cities, the company plans to open 200 standalone "Expo Design Center" stores nationwide, targeting upscale kitchen and bath remodeling customers. The 80,000-square-foot showrooms will feature dozens of elegant displays and provide full installation services. The new Expo stores could eventually dominate the kitchen and bath remodeling industry in the same way Home Depot has rapidly grabbed a leading share of DIY sales in many markets. Construction partner? Although Home Depot does some 20% of its business with pro customers, many contractors find it inconvenient to work with the consumer-oriented mass retailer, which does not offer discounts or