Add material box-shadow to compose area

This increases contrast against the scrollbar.

src/sass/sections/_compose_area.scss

@@ -1,5 +1,9 @@
 compose-area {
 
+    > div {
+        box-shadow: 0 -2px 4px -2px rgba(0, 0, 0, 0.2);
+    }
+
     > div:first-child {
         display: flex;
         flex-direction: row;