Political placards on a homeowner's lawn or in a window of her home can't be prohibited, the Supreme Court unanimously ruled today. The case has limited effect, since it applies only to political speech on private property. But it sets the court firmly against municipalities whose strict codes on neighborhoods might limit a citizen's First Amendment rights. The victor was a St. Louis resident of a posh neighborhood who was opposed to the Persian Gulf War. Her political statement: "For Peace in the Gulf."