Here is the updated code with both features added:
1. **Piano Roll Grid Overlay & Pitch Labels:** A subtle vertical grid background displaying standard musical note labels (`C3` through `C6`) along the screen edge, showing exactly where every pitch lives vertically and how timbre maps horizontally across the canvas.
2. **Fade Canvas Toggle:** The `#fadeCanvasCheck` toggle dynamically fades drawn paths over time when checked.
```html
Harmonic Canvas & Dynamic Audio Visualizer