fix: reduce zoom button size to image dimensions

Reduce the size of the zoom button to the dimensions of the image. Change `flex-direction` of the zoomable container to `column` and align its items `center`. Set `max-width` and `max-height` of the zoom button to `100%` to prevent overflows.

Fixes #5586

stylesheets/components/Lightbox.scss

@@ -131,12 +131,16 @@
 
   &__zoomable-container {
     display: flex;
+    flex-direction: column;
     justify-content: center;
+    align-items: center;
     margin: 0 40px;
   }
 
   &__zoom-button {
     @include button-reset;
+    max-height: 100%;
+    max-width: 100%;
     cursor: zoom-in;
   }