No surprise, but we're so excited about spring and the warm-weather dressing it entails. Sunny days can usher in some trends that aren't exactly work-appropriate, but one of our favorites is so prim and polished that it's basically calling out for the Monday to Friday rotation. The cap-toe slingback, inspired by a retro look that your grandmother probably had in her closet, has been gaining sartorial steam since being the only shoe sent down Chanel's fall 2015 runway. It started popping up…