Define typescript type for placeholders

client/placeholders/Rectangle.tsx

@@ -1,6 +1,10 @@
 import React from 'react';
 
-const Rectangle = ({ height = 8 }) => {
+interface RectangleProps {
+    height?: number;
+}
+
+const Rectangle: React.FC<RectangleProps> = ({ height = 8 }) => {
     return (
         <div
             className="w-100 bg-black-30 br1"