fix(extensions/code): style issues

prevent unexpected overlap

src/main/frontend/extensions/code.css

@@ -1,9 +1,14 @@
 .extensions__code {
-    @apply relative
+    @apply relative;
+    z-index: 0;
 }
 
 .extensions__code-lang {
     @apply absolute top-0 right-0 p-1 text-sm text-gray-500;
-    z-index: 2;
+    z-index: 1;
     background: white;
+}
+
+.extensions__code > .CodeMirror {
+    z-index: 0;
 }