Widen add-extension button to match category.

Goes along with https://github.com/LLK/scratch-blocks/pull/1646

src/components/gui/gui.css

@@ -201,7 +201,7 @@
 }
 
 .extension-button-container {
-    width: 3.25rem;
+    width: 3.75rem;
     height: 3.25rem;
     position: absolute;
     bottom: 0;