Fixed Z depth of sign-in page

frontend/app/sign-in/page.tsx

@@ -8,7 +8,7 @@ const SignInPage = () => {
   return (
     <Main>
       <Header />
-      <div className="rounded-xl shadow-xl p-4 mb-8 z-10 max-w-5xl w-full">
+      <div className="rounded-xl shadow-xl p-4 mb-8 max-w-5xl w-full">
         <div className="max-w-2xl mx-auto text-center">
           <div className="flex flex-col items-center justify-center py-10 space-y-4">
             <div className="bg-blue-500 text-white px-8 py-6 rounded-lg shadow-lg">