fix: deduplicate VolumeSliderItem and add missing stopPropagation

- Extract VolumeSliderItem to voiceMenuItems.tsx (was duplicated in
  VoiceUser.tsx and VoiceChannel.tsx)
- Add e.stopPropagation() to all imperative context menu handlers
  for consistency with useContextMenu hook behavior

packages/web/src/components/voice/StreamTile.tsx

@@ -208,6 +208,7 @@ export function StreamTile({ tile, large }: StreamTileProps) {
   const handleContextMenu = useCallback(
     (e: React.MouseEvent) => {
       e.preventDefault();
+      e.stopPropagation();
 
       const items: ContextMenuItem[] = [];