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/components/ToastHost.vue

@@ -0,0 +1,75 @@
+<template>
+  <div class="toast-host">
+    <transition-group name="toast">
+      <div
+        v-for="t in toast.items"
+        :key="t.id"
+        class="toast"
+        :class="`toast-${t.type}`"
+        @click="toast.dismiss(t.id)"
+      >
+        <span class="toast-message">{{ t.message }}</span>
+        <button class="toast-close" @click.stop="toast.dismiss(t.id)" aria-label="Dismiss">&times;</button>
+      </div>
+    </transition-group>
+  </div>
+</template>
+
+<script setup>
+import { useToast } from '../composables/toast'
+
+const toast = useToast()
+</script>
+
+<style scoped>
+.toast-host {
+  position: fixed;
+  bottom: 1.25rem;
+  right: 1.25rem;
+  z-index: 3000;
+  display: flex;
+  flex-direction: column;
+  gap: 0.6rem;
+  max-width: min(420px, calc(100vw - 2.5rem));
+}
+
+.toast {
+  display: flex;
+  align-items: flex-start;
+  gap: 0.75rem;
+  padding: 0.75rem 0.9rem;
+  border-radius: 8px;
+  background: var(--bg-card);
+  color: var(--text);
+  border-left: 4px solid var(--secondary);
+  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.25);
+  cursor: pointer;
+  font-size: 0.9rem;
+  line-height: 1.35;
+}
+.toast-success { border-left-color: var(--success); }
+.toast-error { border-left-color: var(--danger); }
+.toast-info { border-left-color: var(--primary); }
+
+.toast-message { flex: 1; }
+
+.toast-close {
+  border: none;
+  background: none;
+  color: var(--text-light);
+  font-size: 1.1rem;
+  line-height: 1;
+  cursor: pointer;
+  padding: 0;
+}
+
+.toast-enter-active,
+.toast-leave-active {
+  transition: opacity 0.2s ease, transform 0.2s ease;
+}
+.toast-enter-from,
+.toast-leave-to {
+  opacity: 0;
+  transform: translateX(20px);
+}
+</style>