Implement proper mute/unmute button functionality for 2D controls

- Add CSS classes (.muted/.unmuted) for mute button state management
- Fix button logic to show action that will be taken (standard UI pattern):
  - When audio playing (unmuted): shows mute icon (click to mute)
  - When audio muted: shows unmute icon (click to unmute)
- Add proper hover states for both mute and unmute icons
- Initialize button state correctly on video load
- Smooth 0.15s transitions between all states
- Follows standard media player UI conventions

vr180-player.css

@@ -162,6 +162,22 @@
 	background-image: url(mute-hover.png);
 }
 
+#mute.muted {
+	background-image: url(mute.png);
+}
+
+#mute.muted:hover {
+	background-image: url(mute-hover.png);
+}
+
+#mute.unmuted {
+	background-image: url(unmute.png);
+}
+
+#mute.unmuted:hover {
+	background-image: url(unmute-hover.png);
+}
+
 #nav {
 	grid-area: nav;
 	display: grid;