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/composables/useOnnxPreloader.ts

@@ -0,0 +1,43 @@
+import { ref, readonly } from 'vue'
+
+/**
+ * Idle-preload the Silero VAD ONNX model and its companion assets.
+ *
+ * `@ricky0123/vad-web` pulls ~2MB of ONNX model + ~5MB of onnxruntime-web
+ * WASM the first time `MicVAD.new()` runs. Calling `defaultModelFetcher`
+ * during page idle warms the browser cache so the first mic click feels
+ * instant. If the preload fails, VAD simply loads lazily on first click.
+ */
+const ready = ref(false)
+let loading = false
+
+export function useOnnxPreloader() {
+  async function preload() {
+    if (ready.value || loading) return
+    loading = true
+    try {
+      const { defaultModelFetcher } = await import('@ricky0123/vad-web')
+      // Asset lives at site root because vite-plugin-static-copy drops it
+      // there. Match whatever path MicVAD will use at call time (see useVad).
+      await defaultModelFetcher('/silero_vad_v5.onnx')
+      ready.value = true
+    } catch {
+      // Non-fatal — MicVAD will load the model itself on first use.
+    } finally {
+      loading = false
+    }
+  }
+
+  function schedulePreload() {
+    if (ready.value) return
+    if (typeof window === 'undefined') return
+    if ('requestIdleCallback' in window) {
+      (window as Window & { requestIdleCallback: (cb: () => void, opts: { timeout: number }) => void })
+        .requestIdleCallback(() => void preload(), { timeout: 5000 })
+    } else {
+      setTimeout(() => void preload(), 5000)
+    }
+  }
+
+  return { ready: readonly(ready), schedulePreload }
+}