Remove button focus outline

Removes the black outline that appears when the "시작하기" button is focused.

src/components/onboarding/WelcomeDialog.tsx

@@ -74,7 +74,7 @@ const WelcomeDialog: React.FC<WelcomeDialogProps> = ({ open, onClose }) => {
 
         <DialogFooter className="sm:justify-center">
           <Button 
-            className="w-full sm:w-auto bg-neuro-income text-white hover:bg-neuro-income/90"
+            className="w-full sm:w-auto bg-neuro-income text-white hover:bg-neuro-income/90 focus:outline-none focus:ring-0"
             onClick={onClose}
           >
             시작하기 <ArrowRight className="ml-2 h-4 w-4" />