feat: air-gapped deployment guide, no implicit downloads

- Ship tiktoken's cl100k_base inside the package and build the encoding
  from it, so token counting never downloads anything.
- Default EMBEDDINGS_CACHE_DIR to <data home>/models instead of FastEmbed's
  temp dir, and read tokenizer.json and repo metadata from that cache, so
  a model downloads once and survives reboots.
- TTS_PROVIDER=none and STT_PROVIDER=none switch the speech features off:
  the endpoints return 404, audio files fail to ingest with a clear
  message, /api/config reports tts_available/stt_available, and the UI
  hides the Speak and microphone buttons.
- Drop the Google Fonts Roboto import from the web UI.
- prefetch-models fills the cache the app reads; verify-offline checks the
  packaged encoding.
- Docs: new Air-Gapped Deployment guide, settings and cache notes.

docs/content/Deploying/_meta.js

@@ -27,6 +27,10 @@ export default {
     "title": "☸️ Deploying on Kubernetes",
     "href": "/Deploying/Kubernetes-Deploying"
   },
+  "Air-Gapped": {
+    "title": "🔒 Air-Gapped Deployment",
+    "href": "/Deploying/Air-Gapped"
+  },
   "Hosting-the-app": {
     "title": "☁️ Hosting DocsGPT",
     "href": "/Deploying/Hosting-the-app"