css: standardize box shadows.

* Standardize dialog box and tooltip box shadows.

* Add shadows behind all other floating elements, namely the
  autocomplete menu and translation notes (except for embedded notes,
  which are usually meant to be seamless).

app/javascript/src/styles/common/autocomplete.scss

@@ -1,6 +1,7 @@
 .ui-autocomplete.ui-widget {
   font-size: var(--text-sm);
   border: 1px solid var(--autocomplete-border-color);
+  box-shadow: var(--shadow-lg);
 
   div.ui-menu-item-wrapper {
     padding: 0.25em 0.4em;

app/javascript/src/styles/common/jquery_ui_custom.scss

@@ -24,7 +24,7 @@ div.ui-dialog {
   overflow: visible;
   font-size: 1em;
   z-index: 999 !important;
-  box-shadow: var(--jquery-ui-dialog-box-shadow);
+  box-shadow: var(--shadow-lg);
   padding: 0;
 
   .ui-dialog-titlebar {