Even as a zealous, everyday coffee drinker, I used to think milk frothers were just another food gadget I didn’t need. Fun? Sure. Essential? Meh, not really. My Mr. Coffee got me through the years just fine. Then I tried one, and suddenly my sad splash of half-and-half turned into silky, glossy foam that made my morning drip coffee or espresso feel like a latte from a fancy café. Now I can’t go back. But the milk frother game is tricky. Some whip dairy and nondairy milk into airy soap bubbles that collapse before you even take a sip, while others create the good stuff—smooth, velvety foam that looks (and tastes) luxurious.…