SEOUL, Feb 5 (Reuters) - South Korea's efforts to stabilise its currency face a major obstacle at home - the record appetite of its retail investors for U.S. stocks that is fuelling dollar demand as the won languishes at 17-year lows. The greenback's persistent strength against the won over the past year has complicated Seoul's plans to invest $350 ​billion in U.S. industries under a trade deal with Washington, amid concerns additional outflows could weaken the won further. Those pressures have only be ...