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.

docsgpt/Dockerfile

@@ -108,11 +108,10 @@ COPY --from=builder /venv /venv
 #   EMBEDDINGS_CACHE_DIR / HF_HUB_CACHE  FastEmbed models and their tokenizers
 #                                        (chunking reads tokenizer.json from
 #                                        the same hub-layout snapshot)
-#   TIKTOKEN_CACHE_DIR                   cl100k_base for token accounting
 #   DOCLING_ARTIFACTS_PATH               docling's models (docling extra only)
+# tiktoken's cl100k_base encoding ships inside the package.
 ENV EMBEDDINGS_CACHE_DIR=/app/models \
     HF_HUB_CACHE=/app/models \
-    TIKTOKEN_CACHE_DIR=/app/models/tiktoken \
     DOCLING_ARTIFACTS_PATH=/app/models/docling \
     HF_HUB_DISABLE_TELEMETRY=1 \
     PATH="/venv/bin:$PATH"