FOR years, they have been the showy accessories of the fashion-forward, urban 20-or-30-something set — hipsters, that is — from Brooklyn to San Francisco. They have long been fetishized, too, in Tokyo and London. It has taken more time for the elegant fixed-gear bicycle to catch on in ever-reticent Paris, but the style is now decidedly in vogue. Suddenly, fixies — the finicky one-speed bikes tha…