Refactor layout components and add loading state

- Refactor the layout components to improve code organization and readability.
- Add a new store called LoadingStore to manage the loading state.
- Update AppLayout.vue, AppMenu.vue, AppTopbar.vue, ScenarioStore.js, and UserPrefStore.js to import and use the LoadingStore.
- Add loading indicators in AppTopbar.vue and ScenarioList.vue components.

src/layout/AppMenu.vue

@@ -1,8 +1,11 @@
 <script setup>
 import { ref } from 'vue';
 
+import { UserPrefStore } from '../stores/UserPrefStore.js';
 import AppMenuItem from './AppMenuItem.vue';
 
+const userPrefStore = UserPrefStore();
+
 const model = ref([
     {
         label: 'Scenarios',