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/Development-Environment.mdx

@@ -78,7 +78,7 @@ To run the DocsGPT backend locally, you'll need to set up a Python environment a
 
 3.  **Embedding Model (no action needed):**
 
-    The embedding model is downloaded automatically the first time you ingest a document, and cached for subsequent runs. Set `EMBEDDINGS_CACHE_DIR` to control where.
+    The embedding model is downloaded automatically the first time you ingest a document, and cached under `models/` in the repository root for subsequent runs. Set `EMBEDDINGS_CACHE_DIR` to use another directory.
 
     For an offline or air-gapped machine, fetch it ahead of time instead: