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/stores/toast.js

@@ -1,20 +1,18 @@
-/**
- * Phase 7.1 STUB — Phase 10 (UX-10) fills in the full implementation.
- *
- * The signature `show(message, type, duration)` is locked and Phase 10 must
- * honor it without modifying Phase 8 call sites.
- *
- * Default values match UI-SPEC.md:
- *   type     = 'success'  (NOT 'info')
- *   duration = 4000
- */
+import { ref } from 'vue'
 import { defineStore } from 'pinia'
 
 export const useToastStore = defineStore('toast', () => {
-  // eslint-disable-next-line no-unused-vars
+  const toasts = ref([])
+
   function show(message, type = 'success', duration = 4000) {
-    // No-op stub — Phase 10 implements rendering.
+    const id = Date.now() + Math.random()
+    toasts.value.push({ id, message, type, duration })
+    if (duration > 0) setTimeout(() => dismiss(id), duration)
   }
 
-  return { show }
+  function dismiss(id) {
+    toasts.value = toasts.value.filter(t => t.id !== id)
+  }
+
+  return { toasts, show, dismiss }
 })