Global toast notifications; replace all alert() calls

Add a useToast() composable + a single ToastHost mounted in AppLayout. Convert
every alert() across views/components (29 call sites, all error paths) to
toast.error, and use toast.success to confirm a warranty refresh. Kills the
native "localhost says" dialog and gives consistent, dismissable feedback.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/views/settings/DashboardDefaultsList.vue

@@ -104,6 +104,8 @@
 <script setup>
 import { ref, onMounted } from 'vue'
 import { dashboardDefaultsApi, businessUnitsApi } from '../../api'
+import { useToast } from '../../composables/toast'
+const toast = useToast()
 
 const items = ref([])
 const businessUnits = ref([])
@@ -181,7 +183,7 @@ async function deleteItem() {
     toDelete.value = null
     loadData()
   } catch (err) {
-    alert('Failed to delete')
+    toast.error('Failed to delete')
   }
 }
 </script>