feat(10-04): implement toast store, ToastContainer, mount in App.vue

- toast.js: reactive toasts array, show/dismiss with auto-dismiss via setTimeout
- ToastContainer.vue: Options API, Teleport to body, TransitionGroup, data-test attr
- type-to-class maps: accentClass/iconName/iconColorClass for success/error/warning/info
- App.vue: import and mount <ToastContainer /> after layout div
- all 10 tests pass; SettingsAccountTab and TotpEnrollment regressions green

frontend/src/App.vue

@@ -6,6 +6,7 @@
       <router-view />
     </main>
   </div>
+  <ToastContainer />
 </template>
 
 <script setup>
@@ -13,6 +14,7 @@ import { onMounted } from 'vue'
 import { useRoute } from 'vue-router'
 import AppSidebar from './components/layout/AppSidebar.vue'
 import AuthLayout from './layouts/AuthLayout.vue'
+import ToastContainer from './components/ui/ToastContainer.vue'
 import { useTopicsStore } from './stores/topics.js'
 
 const route = useRoute()