fix: render AddShowButton if user is superuser

The AddShowButton was not rendered when the shows weren’t loaded, but
the only requirement for it to render should be if the user is a
superuser.

src/components/shows/AddShowButton.vue

 <template>
-  <div v-if="loaded.shows">
+  <div v-if="isSuperuser">
     <AddShowModal v-if="!modal" ref="addShowModal" />
 
     <b-button
-      v-if="authStore.isSuperuser"
       variant="primary"
       data-testid="show-selector:add-show"
       class="md:tw-whitespace-nowrap"
@@ -31,13 +30,12 @@ const props = defineProps<{
 }>()
 
 const store = useStore()
-const authStore = useAuthStore()
+const { isSuperuser } = useAuthStore()
 const { t } = useI18n()
 
 const addShowModal = ref<Modal>()
 const resolvedModal = computed(() => (props.modal ?? addShowModal) as unknown as Modal)
 const loaded = computed(() => ({
-  shows: store.state.shows.loaded.shows,
   types: store.state.shows.loaded.types,
   fundingCategories: store.state.shows.loaded.fundingCategories,
 }))