💄 style(avatar): update background opacity syntax

- replace bg-black bg-opacity-50 with bg-black/50 for more concise Tailwind syntax

src/client/mobile/components/AvatarCropper.tsx

@@ -188,7 +188,7 @@ export const AvatarCropper: React.FC<AvatarCropperProps> = ({
   };
 
   return (
-    <div className="fixed inset-0 bg-black bg-opacity-50 flex items-center justify-center z-50 p-4">
+    <div className="fixed inset-0 bg-black/50 flex items-center justify-center z-50 p-4">
       <div 
         className="rounded-2xl p-6 max-w-md w-full"
         style={{