VAD: Silero v5 (vad-web) statt RMS-Eigenbau — Turn-Ende 900 -> 450 ms (Review P1-7)

- useVAD.ts: MicVAD (neuronale Sprach- statt Pegel-Erkennung), redemptionMs 450,
  preSpeechPad 320 ms, WAV direkt aus Float32 (kein webm/Opus-Umweg mehr zur Box)
- boxStt.ts: Dateiname passend zum Blob-Typ (rec.wav | rec.webm)
- Assets self-hosted unter /vad/ via scripts/copy-vad-assets.mjs (npm postinstall), offlinefaehig
- Verifiziert: tsc + Build sauber, App startet, Assets werden ausgeliefert (Mikro-Test: User)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

client/lucy-desktop/package.json

@@ -7,13 +7,15 @@
   "scripts": {
     "dev": "electron-vite dev",
     "build": "electron-vite build",
-    "preview": "electron-vite preview"
+    "preview": "electron-vite preview",
+    "postinstall": "node scripts/copy-vad-assets.mjs"
   },
   "dependencies": {
     "@pixiv/three-vrm": "^3.4.0",
     "@pixiv/three-vrm-animation": "^3.5.4",
     "@react-three/drei": "^9.114.0",
     "@react-three/fiber": "^8.17.10",
+    "@ricky0123/vad-web": "^0.0.30",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-markdown": "^9.1.0",