COPENHAGEN — Danish Prime Minister Mette Frederiksen’s Social Democrats are staring down a potential political earthquake in Tuesday’s nationwide local elections. Polls predict a drubbing in the very cities that once anchored the party’s power. But the biggest humiliation may come in Copenhagen, where the Social Democrats are poised to lose control of city hall for the first time in 122 years. The revolt is driven by a familiar urban grievance: The skyrocketing cost of housing . After decades