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/warranty/WarrantiesList.vue

@@ -5,6 +5,7 @@
       <button class="btn btn-primary" @click="openModal()">+ Add Warranty</button>
     </div>
 
+
     <div class="filters">
       <label>Status
         <select v-model="statusFilter" class="form-control" @change="loadData">
@@ -138,6 +139,9 @@
 import { ref, onMounted } from 'vue'
 import { colorStyle } from '@/utils/colorStyle'
 import { warrantyApi, assetsApi } from '../../api'
+import { useToast } from '../../composables/toast'
+
+const toast = useToast()
 
 const items = ref([])
 const loading = ref(true)
@@ -261,7 +265,7 @@ async function deleteWarranty(w) {
     await warrantyApi.remove(w.warrantyid)
     loadData()
   } catch (err) {
-    alert(apiError(err, 'Failed to delete'))
+    toast.error(apiError(err, 'Failed to delete'))
   }
 }
 
@@ -270,9 +274,11 @@ async function refresh(w) {
     const response = await warrantyApi.refresh(w.warrantyid)
     loadData()
     const updated = response.data?.data
-    if (updated) alert(`Refreshed: ${updated.vendor} - ${updated.servicelevel || 'coverage'} ends ${updated.enddate || 'unknown'}`)
+    if (updated) {
+      toast.success(`${updated.vendor}: ${updated.servicelevel || 'coverage'} ends ${updated.enddate || 'unknown'}`)
+    }
   } catch (err) {
-    alert(apiError(err, 'Refresh failed'))
+    toast.error(apiError(err, 'Refresh failed'))
   }
 }
 </script>