Roughly 17.3 percent of office space in Manhattan is available for lease, the most in at least three decades. Asking rents have dropped to just over $74 a square foot, from nearly $82 at the beginning of 2020, according to the real estate services company Newmark. Elsewhere, asking rents are largely flat from a year ago, including in Boston and Houston, but have climbed slightly in Chicago.The Japanese clothing brand Uniqlo, whose United States headquarters are in Manhattan’s SoHo neighborhood, recently relocated to another building nearby, an open layout with tables designed for 130 people who will come into the office only a …