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.

tests/stt/test_stt_creator.py

@@ -51,6 +51,16 @@ def test_stt_type_case_insensitivity(stt_creator):
         assert result == instance
 
 
+@pytest.mark.parametrize("value", ["none", "NONE", " none ", "", None])
+def test_is_enabled_false_when_switched_off(value):
+    assert STTCreator.is_enabled(value) is False
+
+
+@pytest.mark.parametrize("value", ["openai", "faster_whisper"])
+def test_is_enabled_for_a_provider(value):
+    assert STTCreator.is_enabled(value) is True
+
+
 def test_stt_providers_integrity(stt_creator):
     providers = stt_creator.stt_providers
     assert "openai" in providers