BERLIN — Leaders of an extremist, far-right party in Germany have quietly borrowed pages from President Trump’s playbook to cast doubt on the integrity of absentee voting — alleging without evidence that residents of nursing homes have had their ballots manipulated and calling for an end to postal voting.The attack on mail ballots comes as Germany approaches a potentially historic state election that could shatter one of the taboos of its postwar political order by opening the door to a far-right government. Politicians across the ideological spectrum, mindful that polls show Trump to be widely unpopular, don’t invoke his name. But...