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/CustomFieldsList.vue

@@ -114,6 +114,8 @@
 <script setup>
 import { ref, computed, onMounted } from 'vue'
 import { assetsApi, customFieldsApi } from '../../api'
+import { useToast } from '../../composables/toast'
+const toast = useToast()
 
 const assetTypes = ref([])
 const assettypeid = ref(null)
@@ -206,7 +208,7 @@ async function deleteField(f) {
     await customFieldsApi.remove(f.fieldid)
     loadFields()
   } catch (err) {
-    alert(err.response?.data?.error?.message || err.response?.data?.message || 'Failed to delete')
+    toast.error(err.response?.data?.error?.message || err.response?.data?.message || 'Failed to delete')
   }
 }
 </script>