playground: use app.head instead of useHead

playground/nuxt.config.ts

@@ -1,4 +1,11 @@
 // https://nuxt.com/docs/api/configuration/nuxt-config
 export default defineNuxtConfig({
+  app: {
+    head: {
+      bodyAttrs: {
+        class: 'antialiased font-sans text-gray-700 dark:text-gray-200 bg-white dark:bg-gray-900'
+      }
+    }
+  },
   modules: ['../src/module']
 })