Need a bandeau bikini, a best-selling biography or bag charms? The East End’s newest pop-ups and stores have you covered. Here’s a peek at some of the establishments slated for this season, from an engrossing bookshop in Southampton and a glam Chanel boutique in East Hampton to a tween destination in Sag Harbor.  CHANELMANIA With new artistic director Matthieu Blazy churning out hits, Chanel is dominating fashion conversations again. The French maison took the prime 17 Newtown Lane address in