UX: Show muted audio icon when volume reaches 0%

Currently, when the system volume is lowered to 0% using keyboard media keys, mouse wheel scrolling on the volume control, or by dragging the volume slider in Settings, the audio icon continues to show the normal speaker icon with 0%.

https://github.com/user-attachments/assets/c55c73a8-2598-468d-9d63-32d21131b1d2

Start by locating the volume control and audio icon entry points in the desktop shell, then trace changes from keyboard media keys, mouse-wheel scrolling, and the Settings volume slider. Done means the icon shows the muted variant whenever volume reaches 0% through each of these paths, while remaining normal above 0%.

Written by the indexing model from the issue text.