Change Pinia value before initial render

/store/site.ts
--------------

export const useSiteStore = defineStore('site', () => {
  const breadcrumbs = ref(false)

  return {
    breadcrumbs
  }
}

/pages/[slug].vue
-----------------

<script setup>
import { useSiteStore } from '~/store/site'

const { breadcrumbs } = storeToRefs(useSiteStore())

const page = await useFetch('<API endpoint>', { slug: useRoute().params.slug })

if (page.showBreadcrumbs) {
  breadcrumbs.value = true
}
</script>

app.vue
-------

<template>
  <Header />
  <Breadcrumbs v-if="breadcrumbs" />
  <NuxtPage />
  <Footer />
</template>

<script setup>
import { useSiteStore } from '~/store/site'

const { breadcrumbs } = storeToRefs(useSiteStore())
</script>