BEIRUT — A cease-fire came into effect in Syria at sunset Monday in the latest attempt led by the United States and Russia to bring some quiet in the 5 1/2 -year civil war. Residents and observers reported quiet in most of the country hours after the truce came into effect, though activists said airstrikes took place on contested areas around the northern city of Aleppo. But the most powerful rebel groups have shown deep misgivings over the cease-fire deal, which was crafted without their input last weekend in Geneva between the top U.S.