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/AppLayout.vue

@@ -69,12 +69,14 @@
       </div>
       <router-view />
     </main>
+    <ToastHost />
   </div>
 </template>
 
 <script setup>
 import { ref, onMounted } from 'vue'
 import { useRouter } from 'vue-router'
+import ToastHost from '../components/ToastHost.vue'
 import {
   Sun, Moon, LayoutDashboard, Calendar, Map, Cog, Monitor,
   Printer, Globe, Usb, AppWindow, BookOpen, BarChart3, Bell, Image, ShieldCheck