Fix toast rendering: display toast.msg instead of object

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/App.vue

@@ -64,7 +64,7 @@
   <CustomDialog />
 
   <!-- Toast messages -->
-  <div v-for="(toast, i) in ui.toasts" :key="i" class="toast">{{ toast }}</div>
+  <div v-for="toast in ui.toasts" :key="toast.id" class="toast">{{ toast.msg }}</div>
 </template>
 
 <script setup>