fix(exam): fix pretest screen start exam button

components/Header.tsx

@@ -4,6 +4,7 @@ import Image from "next/image";
 import { ChevronLeft, Layers } from "lucide-react";
 import { useTimer } from "@/context/TimerContext";
 import styles from "@/css/Header.module.css";
+import { useExam } from "@/context/ExamContext";
 
 const API_URL = "https://examjam-api.pptx704.com";
 
@@ -26,6 +27,7 @@ const Header = ({
   examDuration,
 }) => {
   const router = useRouter();
+  const { clearExam } = useExam();
   const [totalSeconds, setTotalSeconds] = useState(
     examDuration ? parseInt(examDuration) * 60 : 0
   );
@@ -86,6 +88,7 @@ const Header = ({
       if (stopTimer) {
         stopTimer();
       }
+      clearExam();
       router.push("/unit");
     }
   };