HELSINKI, April 2 (Reuters) - Finland's left-wing Prime Minister Sanna Marin conceded defeat on Sunday in the Nordic country's parliamentary election as the opposition right-wing National Coalition Party (NCP) claimed victory in a tightly fought contest. Marin, 37, the world's youngest prime minister when she took office in 2019, is considered by fans around the globe as a millennial role model for progressive new leaders, but at home she has faced criticism for her partying and her government's public spending. Most notable of Marin's foreign policy actions has been her push, along with President Sauli Niinisto, for the country to make a watershed policy U-turn by seeking NATO membership in the wake of Russia's invasion of Ukraine