Add audio mute toggle to the modal player

[sausi-7]

The modal opens games in an iframe but there's no global mute. Add a 🔊 / 🔇 button next to the fullscreen / restart buttons.

Approach

• Add the button in index.html modal toolbar.
• Use postMessage to the iframe so individual games can opt in (most won't), but as a fallback set iframe.allow="autoplay 'none'" or use the muted attribute trick.
• Persist the preference in localStorage.

Good first issue if you've worked with iframe messaging before.