refactor: 可読性のため一部でArray.prototype.atを使うように (#11274)

* refactor: `Array.prototype.at`を使うように

* fixup! refactor: `Array.prototype.at`を使うように

packages/frontend/src/components/MkPageWindow.vue

@@ -120,7 +120,7 @@ const contextmenu = $computed(() => ([{
 
 function back() {
 	history.pop();
-	router.replace(history[history.length - 1].path, history[history.length - 1].key);
+	router.replace(history.at(-1)!.path, history.at(-1)!.key);
 }
 
 function reload() {