style(muscle-selection.tsx): add scale effect on active muscle illustration for better user interaction feedback

src/features/workout-builder/ui/muscle-selection.tsx

@@ -39,6 +39,7 @@ const MuscleIllustration = ({
       "cursor-pointer transition-colors",
       isSelected ? "fill-primary" : "fill-[#757575]",
       "md:group-hover:fill-primary/80 group-active:fill-primary/70 focus-visible:outline focus-visible:outline-2 focus-visible:outline-primary",
+      "group-active:scale-[0.99]",
     );
   };