If the words “sleeper sofa” still conjure images of a musty, squeaky, lumpy pull-out mattress with the thickness and support of a peanut butter sandwich, you may want to take a look at what’s been going on in the world of convertible furniture lately. Modern-day sleeper sofas now come with luxe, real mattresses, like the Tempur-Pedic in Joybird’s Eliot, or offer multiple sitting and sleeping configurations, like the Koala Wanda. The Neptune, from Montreal-based, direct-to-consumer home goods brand Cozey, aims for something entirely different: a couch with modular components that can be moved or added to, along with seats that pull out to make either a twin- or full-sized bed, depending on couch size. Add on storage underneath each seat and machine-washable fabric, and you’ve got a truly versatile setup that’s ideal for guest rooms and living rooms alike. That is, as long as you like things firm. Build-a-Bed Cozey offers a variety of Neptune packages, from chairs and loveseats to five-seat sectionals and everything in between. Bed size options include twin or full, with optional storage …