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/.gitignore

@@ -0,0 +1,11 @@
+.env
+*.pyc
+__pycache__/
+*.log
+qwen_tts.db
+voice_cache/
+outputs/
+venv/
+.pytest_cache/
+.coverage
+htmlcov/