remove utils

* add ~platform/DocumentTitle
* add titles for some pages

src/platform/DocumentTitle.tsx

@@ -0,0 +1,22 @@
+import { children, createRenderEffect, onCleanup, type JSX } from "solid-js";
+
+/**
+ * Document title.
+ *
+ * The `children` must be plain text.
+ */
+export default function (props: { children?: JSX.Element }) {
+  let otitle: string | undefined;
+
+  createRenderEffect(() => (otitle = document.title));
+
+  const title = children(() => props.children);
+
+  createRenderEffect(
+    () => (document.title = (title.toArray() as string[]).join("")),
+  );
+
+  onCleanup(() => (document.title = otitle!));
+
+  return <></>;
+}