Replace dropdown shadow with outset-style border

Shadows don't really fit with the site's overall style, and looked
especially weird in dark themes.

tildes/scss/modules/_menu.scss

@@ -2,6 +2,9 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
 .menu {
+  box-shadow: none;
+  border: 1px outset;
+
   .menu-item {
     > a:hover,
     > a:focus {

tildes/scss/themes/_theme_base.scss

@@ -401,7 +401,7 @@
 
   .menu {
     background-color: map-get($theme, "background-primary");
-    box-shadow: 0 0.1rem 0.2rem map-get($theme, "foreground-secondary");
+    border-color: map-get($theme, "border");
   }
 
   .message {