WASHINGTON — States can't demand proof of citizenship from people registering to vote in federal elections unless they get federal or court approval to do so, the Supreme Court ruled Monday in a decision complicating efforts in Arizona and other states to bar voting by people who are in the country illegally. The justices' 7-2 ruling closes the door on states independently changing the requirements for those using the voter-registration form produced under the federal "motor voter" registration law. They would need permission from a federally created panel, the Election Assistance Commission, or a federal court ruling overturning the commission's decision, to make tougher requirements stick.