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/package.json

@@ -36,6 +36,7 @@
     "@vitejs/plugin-vue": "^6.0.0",
     "typescript": "~5.9.0",
     "vite": "^7.0.0",
+    "vite-plugin-static-copy": "^4.0.1",
     "vue-tsc": "^3.0.0"
   }
 }