refactor(utils): extract shared helpers into visualnovel/utils.py

fix(TransformersLLM): strip Qwen3 think-tags, add LLM logging, atomic

fix(tts): handle empty audio from kokoro_onnx.create() gracefully

chore(reformat): reformat existing code with ruff.

Auto-download Kokoro model files if missing at startup

Update TTS with "voice description mapper"

Add TTS