feat: add custom 404 error page

Author: kevinlee-06

app/error.vue

@@ -0,0 +1,41 @@
+<script setup>
+const router = useRouter()
+const { currentRoute } = router
+const currentPath = currentRoute.value.fullPath
+const targetUrl = `https://to.ntut.club${currentPath}`
+// async function checkUrl() {
+//   const response = await fetch(targetUrl, { method: 'GET' });
+//   if(response.ok){
+//     console.log(targetUrl);
+//     window.location.href = targetUrl;
+//   }
+// }
+
+// onMounted(() => {
+//   checkUrl();
+// })
+</script>
+
+<template>
+  <div class="error-page">
+    <h1>404 - 找不到頁面</h1>
+    <p>抱歉，<strong>小N</strong>找不到您所請求的頁面：<br><a href="#">https://ntut.club{{ currentPath }}</a></p>
+    <NuxtLink to="/">
+      返回首頁
+    </NuxtLink>
+    <br>
+    <br>
+    <NuxtLink :to="targetUrl">
+      <button>
+        嘗試在 <strong>Shorts 短網址</strong>尋找此路徑
+      </button>
+    </NuxtLink>
+  </div>
+</template>
+
+<style scoped>
+  .error-page {
+  text-align: center;
+  padding: 4rem;
+}
+</style>