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/stt/stt_creator.py

@@ -1,7 +1,12 @@
+from typing import Optional
+
 from docsgpt.stt.base import BaseSTT
 from docsgpt.stt.faster_whisper_stt import FasterWhisperSTT
 from docsgpt.stt.openai_stt import OpenAISTT
 
+#: STT_PROVIDER value that switches speech-to-text off.
+DISABLED = "none"
+
 
 class STTCreator:
     stt_providers = {
@@ -9,6 +14,11 @@ class STTCreator:
         "faster_whisper": FasterWhisperSTT,
     }
 
+    @staticmethod
+    def is_enabled(stt_type: Optional[str]) -> bool:
+        """False when the provider is ``none`` or empty: speech-to-text is switched off."""
+        return (stt_type or "").strip().lower() not in ("", DISABLED)
+
     @classmethod
     def create_stt(cls, stt_type, *args, **kwargs) -> BaseSTT:
         stt_class = cls.stt_providers.get(stt_type.lower())