Do not export an instantiated store - instead export the `createStore` method and manage the instance in `App.vue`

Description of the proposal

We should establish best practices for exporting Vuex stores. We have some instances where we export default createStore().

I propose we only export the createStore method from our stores/index.js files, and never export an instantiated store. Then we manage the instance in the App.vue.

Our example here has export default createStore() - so I hope we can elaborate on that documentation after resolving this discussion.

• Mention the proposal in the next frontend weekly call and the #frontend channel to encourage contribution
• Proceed with the proposal once 50% of the maintainers have weighed in, and 80% of the votes are 👍
• Once approved, mention it again in the next frontend weekly call and the #frontend channel