fix(dictation): self-host Silero VAD / onnxruntime-web assets

Streaming dictation failed at runtime with "no available backend found /
'text/html' is not a valid JavaScript MIME type": @ricky0123/vad-web 0.0.30
defaults baseAssetPath/onnxWASMBasePath to "./" (relative to the page URL),
so the worklet, Silero model and ORT wasm/mjs were requested against the SPA
catch-all and came back as index.html.

Serve them from a fixed /vad/ instead:
- scripts/copy-vad-assets.mjs copies the 4 runtime assets (vad worklet,
  silero_vad_v5.onnx, ort-wasm-simd-threaded.jsep.{mjs,wasm}) from node_modules
  into apps/client/public/vad/ (gitignored — the ORT wasm is ~26 MB)
- client dev/build scripts run the copy first so the assets are always present
- useStreamingDictation points both path constants at "/vad/"

Verified: dev server serves all four under /vad/ with HTTP 200 and correct
Content-Type (js/wasm, never text/html); tsc clean. Prod (Docker) build runs
the copy step, so dist/vad/* ships in the image.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/package.json

@@ -3,8 +3,8 @@
   "private": true,
   "version": "0.93.0",
   "scripts": {
-    "dev": "vite",
-    "build": "tsc && vite build",
+    "dev": "node scripts/copy-vad-assets.mjs && vite",
+    "build": "node scripts/copy-vad-assets.mjs && tsc && vite build",
     "lint": "eslint .",
     "preview": "vite preview",
     "format": "prettier --write \"src/**/*.tsx\" \"src/**/*.ts\"",