1. Add a selectedPostDetails data property in the state and add the corresponding getter and mutation:

      state () {
        return {
          // ...
          // Fetched details for the selected post
          selectedPostDetails: null,
        }
      },

      getters: {
        // ...
        selectedPostDetails: state => state.selectedPostDetails,
      },

      mutations: {
        // ...
        selectedPostDetails (state, value) {
          state.selectedPostDetails = value
        },
      },

2. In the selectPost, fetch the details with a request to the /post/<id> route on the server:

      async selectPost ({ commit }, id) {
        commit('selectedPostDetails', null)
        commit('selectedPostId', id)
        const details = await $fetch(`posts/${id}`)
        commit('selectedPostDetails', details)
      },

3. Also add a new unselectPost action:

      unselectPost ({ commit }) {
        commit('selectedPostId', null)
      },