chore: Update dropdown z-index to be on top of contact details pane (#3411)

app/javascript/dashboard/assets/scss/plugins/_dropdown.scss

@@ -3,7 +3,7 @@
   @include border-light;
   box-sizing: content-box;
   width: fit-content;
-  z-index: 999;
+  z-index: var(--z-index-very-high);
 
   &.dropdown-pane--open {
     display: block;

app/javascript/shared/assets/stylesheets/z-index.scss

@@ -8,5 +8,6 @@
   --z-index-high: 1000;
   --z-index-higher: 2000;
   --z-index-much-higher: 5000;
+  --z-index-very-high: 9999;
   --z-index-highest: 10000;
 }