Feature Request: Add Sound and Music Controls

Some players prefer playing without sound or with their own music, but the game currently lacks audio control options. Without mute buttons, users may find the experience intrusive or distracting.

### 🎯 Proposed Solution
- Add **sound and music toggles** in the settings panel.
- Allow players to control separately:
- Sound effects (e.g., merges, moves).
- Background music (if available).
- Save preferences to local storage for persistence.

### ✅ Expected Behavior
- Players can mute/unmute sound effects and background music independently.
- Audio settings persist across sessions.
- No background audio plays after muting.

### 💻 Technical Notes
- Use HTML5 Audio API or Web Audio API for control.
- Implement volume and mute state management.
- Ensure accessibility: provide visual indicators for mute state.

### 🧪 Test Steps
1. Mute sound effects → verify no sounds play during moves.
2. Mute music → background audio stops.
3. Refresh page → preferences persist.