refactor: rename backend/frontend dirs and remove NovelWriter submodule

- Rename qwen3-tts-backend → canto-backend
- Rename qwen3-tts-frontend → canto-frontend
- Remove NovelWriter embedded repo

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

canto-backend/indextts/s2mel/dac/nn/__init__.py

@@ -0,0 +1,3 @@
+from . import layers
+from . import loss
+from . import quantize