To understand today’s penny-pinching Chinese consumers, behold the vicious price war playing out among the country’s coffee store giants. Luckin Coffee, a popular Chinese chain, rose to prominence and opened 10,800 stores by successfully undercutting Starbucks’s prices. But now, Cotti Coffee, an upstart rival started by the same two people who founded Luckin, is undercutting Luckin’s prices. Cotti has opened stores near Luckin shops, and it is charging — in some cases — 1 yuan, or 15 cents, less than its rival for the same drink. Earlier this year, Cotti…