United Airlines passengers across the country found themselves stranded, confused, and angry on Wednesday as a massive technology failure triggered a nationwide grounding of flights. The culprit: Unimatic, a decades-old internal system so vital to United’s operations that when it failed, the airline had no choice but to bring the entire operation to a standstill.From major hubs in Chicago, Newark, Denver, Houston, to San Francisco, the outage rippled through the airline’s scheduling, flight trac...