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/vendors/VendorsList.vue

@@ -186,6 +186,8 @@
 import { ref, onMounted } from 'vue'
 import { vendorsApi } from '../../api'
 import PaginationBar from '../../components/PaginationBar.vue'
+import { useToast } from '../../composables/toast'
+const toast = useToast()
 
 const vendors = ref([])
 const loading = ref(true)
@@ -321,7 +323,7 @@ async function deleteVendor() {
     loadVendors()
   } catch (err) {
     console.error('Error deleting vendor:', err)
-    alert('Failed to delete vendor')
+    toast.error('Failed to delete vendor')
   }
 }
 </script>