Labor Day marks the line in the corporate sand. Many company leaders say the end-of-summer holiday represents the best chance to finally lean on workers to return to the office this year. After months of encouraging white-collar employees to return, or attempting to coax them back with free pizza, warm cookies and catered lunches, many executives now say they feel emboldened to take a tougher stance. No longer can workers merely come to the office if they so choose; this fall, executives say, attendance is expected and the office resisters will be put on notice. ... ...After Spotify offered most...