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/scripts/verify_offline.py

@@ -21,7 +21,7 @@ import time
 from typing import Callable, List, Optional, Sequence
 
 from docsgpt.core.optional_deps import is_available
-from docsgpt.scripts.prefetch_models import DEFAULT_MODELS, TIKTOKEN_ENCODINGS
+from docsgpt.scripts.prefetch_models import DEFAULT_MODELS
 from docsgpt.vectorstore.model_registry import resolve
 
 logger = logging.getLogger("verify_offline")
@@ -50,16 +50,12 @@ def verify(models: Sequence[str]) -> bool:
     """Run every check; return whether all passed."""
     ok = True
 
-    def tiktoken_check(encoding: str) -> Callable[[], object]:
-        def run() -> object:
-            import tiktoken
+    def tiktoken_check() -> object:
+        from docsgpt.utils import get_encoding
 
-            return f"{len(tiktoken.get_encoding(encoding).encode('hello world'))} tokens"
+        return f"{len(get_encoding().encode('hello world'))} tokens"
 
-        return run
-
-    for encoding in TIKTOKEN_ENCODINGS:
-        ok &= _check(f"tiktoken {encoding}", tiktoken_check(encoding))
+    ok &= _check("tiktoken cl100k_base", tiktoken_check)
 
     for name in models:
         spec = resolve(name)