Greece closed its banks and imposed capital controls on Sunday to check the growing strains on its crippled financial system, bringing the prospect of being forced out of the euro into plain sight. After bailout talks between the leftwing government and foreign lenders broke down at the weekend, the European Central Bank froze vital funding support to Greece’s banks, leaving Athens with little choice but to shut down the system to keep the banks from collapsing. Capital controls are likely to last for many months at least.