add the visitor API endpoint

app/src/lib/error.svelte

@@ -0,0 +1,61 @@
+<script>
+  import Link from "$lib/link.svelte";
+  import { color } from "$lib/util.js";
+  import { _ } from "svelte-i18n";
+
+  export let error = "";
+</script>
+
+<main>
+  <h1 style="color: var(--{color()})">{$_("error.title")}</h1>
+  <code>
+    {#if error === ""}
+      Unknown error
+    {:else}
+      {error}
+    {/if}
+  </code>
+  <Link link={import.meta.env.VITE_BUG_REPORT_URL}>
+    {$_("error.report")}
+  </Link>
+  <img src="/profile/sad.png" alt="" />
+</main>
+
+<style>
+  main {
+    position: fixed;
+
+    top: 0;
+    left: 0;
+    right: 0;
+    bottom: 0;
+    z-index: 1;
+
+    display: flex;
+    flex-direction: column;
+    justify-content: end;
+    gap: 10px;
+
+    padding: 50px;
+    font-size: var(--size-4);
+
+    background: var(--background);
+    background-size: 50%;
+  }
+
+  main h1 {
+    font-size: var(--size-6);
+  }
+
+  main code {
+    font-size: var(--size-4);
+    color: var(--white-2);
+  }
+
+  main img {
+    width: var(--profile-size);
+    position: absolute;
+    right: 0;
+    bottom: 0;
+  }
+</style>