chore: add store/index.js

packages/playground/ssr/src/main.js

 import { createSSRApp } from 'vue'
 import App from './App.vue'
-import createStore from './store/index'
+import createStore from './store'
 export function createApp() {
   const app = createSSRApp(App)
 

packages/playground/ssr/src/store/index.js

+import { createStore } from 'vuex'
+
+// mock
+function fetchItem(id) {
+  return new Promise((resolve) => {
+    setTimeout(() => {
+      resolve({
+        id,
+        title: 'title' + id,
+      })
+    }, 300)
+  })
+}
+
+export default () => {
+  return createStore({
+    state() {
+      return {
+        items: {},
+      }
+    },
+    actions: {
+      fetchItem({ commit }, id) {
+        return fetchItem(id).then((item) => {
+          commit('setItem', { id, item })
+        })
+      },
+    },
+    mutations: {
+      setItem(state, { id, item }) {
+        state.items[id] = item
+      },
+    },
+  })
+}