feat: complete ANL lab homepage with light/dark theme, responsive layout, and 100% SSG E2E test suite

refer_landing_page/app/not-found.tsx

@@ -0,0 +1,30 @@
+import React from "react";
+import Link from "next/link";
+
+export default function NotFound() {
+  return (
+    <div className="container-content py-20 sm:py-32 flex flex-col items-center justify-center text-center min-h-[60vh]">
+      <div className="border border-line bg-paper p-8 sm:p-12 rounded-lg max-w-xl w-full space-y-6 shadow-sm">
+        <div className="inline-block px-3 py-1 bg-vermillion/10 text-vermillion font-mono text-xs font-bold rounded tracking-widest uppercase">
+          404 Error
+        </div>
+        <h1 className="font-serif text-3xl sm:text-4xl font-normal text-ink leading-tight">
+          페이지를 찾을 수 없습니다
+        </h1>
+        <p className="text-ink-mute text-sm sm:text-base leading-relaxed">
+          요청하신 페이지가 존재하지 않거나 이전되었을 수 있습니다.
+          <br />
+          입력하신 주소가 올바른지 다시 한번 확인해 주시기 바랍니다.
+        </p>
+        <div className="pt-4">
+          <Link
+            href="/"
+            className="inline-flex items-center justify-center px-6 py-3 border border-ink bg-ink text-ivory hover:bg-vermillion hover:border-vermillion transition-colors duration-200 text-sm font-medium rounded-md"
+          >
+            메인 페이지로 돌아가기
+          </Link>
+        </div>
+      </div>
+    </div>
+  );
+}