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/composables/toast.js

@@ -0,0 +1,35 @@
+// Global toast notifications. Import useToast() anywhere and call
+// toast.success('...') / toast.error('...') / toast.info('...'). A single
+// <ToastHost /> mounted in AppLayout renders the stack.
+import { reactive } from 'vue'
+
+const state = reactive({
+  items: [],
+})
+
+let nextId = 1
+
+function dismiss(id) {
+  const index = state.items.findIndex(t => t.id === id)
+  if (index !== -1) state.items.splice(index, 1)
+}
+
+function push(message, type = 'info', duration = 5000) {
+  if (!message) return
+  const id = nextId++
+  state.items.push({ id, message, type })
+  if (duration > 0) {
+    setTimeout(() => dismiss(id), duration)
+  }
+  return id
+}
+
+export function useToast() {
+  return {
+    items: state.items,
+    dismiss,
+    success: (message, duration) => push(message, 'success', duration),
+    error: (message, duration) => push(message, 'error', duration ?? 8000),
+    info: (message, duration) => push(message, 'info', duration),
+  }
+}