Replace torchaudio.save with soundfile.write; add EPUB loader node

- nodes/generator.py: swap torchaudio.save for soundfile.write to avoid
  torchcodec/FFmpeg dependency crash in environments without FFmpeg shared libs
- nodes/epub_loader.py: new OmniVoiceEpubLoader node for loading EPUB chapters
- tests/test_epub_loader.py: 8 tests for the EPUB loader
- install.py: add beautifulsoup4 to runtime deps
- __init__.py, nodes/__init__.py: register OmniVoiceEpubLoader

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

install.py

@@ -25,6 +25,7 @@ pip(
     "pydub",
     "soundfile",
     "numpy",
+    "beautifulsoup4",
 )
 
 print("\n[ComfyUI-Omnivoice] Installation complete.")