BERLIN — Syriza, the radical leftist coalition that may win Sunday’s Greek election, is riding a current of vigorous outrage. It vows to call the EU’s bluff, which it claims will allow the heavily indebted country to remain in the euro zone and to keep its bailout billions, while no longer respecting the harsh terms imposed by creditors. Is the party — and Greece — living a dangerous fantasy that could sink the global economy? Whether Syriza is in office or in the streets, the only certainty is mayhem.