BEIRUT/GENEVA (Reuters) - Rebels seized control of one of Syria's largest helicopter bases on Friday, opposition sources said, in their first capture of a military airfield used by President Bashar al-Assad's forces. Fighting raged across the country as international mediator Lakhdar Brahimi sought a political solution to Syria's civil war, meeting senior U.S. and Russian officials in Geneva. But the two world powers are still deadlocked over Assad's fate in any transition. ...