feat(voice): add ONNX WASM idle preloader for Silero VAD

Configures vite-plugin-static-copy to serve @ricky0123/vad-web's ONNX model,
audio worklet, and onnxruntime-web WASM files from the site root.
useOnnxPreloader warms the model cache during page idle so the first mic
click is instant.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/App.vue

@@ -5,6 +5,7 @@ import AppHeader from "@/components/AppHeader.vue";
 import ToastNotification from "@/components/ToastNotification.vue";
 import { useTheme } from "@/composables/useTheme";
 import { useShortcuts } from "@/composables/useShortcuts";
+import { useOnnxPreloader } from "@/composables/useOnnxPreloader";
 import { useAuthStore } from "@/stores/auth";
 import { useChatStore } from "@/stores/chat";
 import { useSettingsStore } from "@/stores/settings";
@@ -12,6 +13,9 @@ import { apiGet, apiPut } from "@/api/client";
 
 useTheme();
 
+const { schedulePreload: scheduleVadPreload } = useOnnxPreloader();
+scheduleVadPreload();
+
 const router = useRouter();
 const appVersion = ref("dev");
 const authStore = useAuthStore();