Retailers are kicking off the unofficial start of the holiday shopping season on Friday with a bevy of discounts and other enticements. But executives are growing concerned with a spending slowdown that could temper sales on the day after Thanksgiving as well as throughout the holidays. Shoppers, powered by a solid job market and steady wage growth, had demonstrated a resilience that confounded economists and ran counter to sour sentiments that Americans themselves have expressed in opinion polls. Such spending came despite higher prices in the grocery aisle and higher borrowing costs. But con...