FIX: Sidebar scroll fade on older iOS (#18299)

* DEV: Use explicit transparent color rather than keyword
* DEV: Use variable for dark mode
* DEV: Add mobile style

app/assets/stylesheets/common/base/sidebar-footer.scss

@@ -24,7 +24,7 @@
       pointer-events: none;
       background: linear-gradient(
         to bottom,
-        transparent,
+        rgba(var(--primary-very-low-rgb), 0),
         rgba(var(--primary-very-low-rgb), 1)
       );
     }

app/assets/stylesheets/mobile/menu-panel.scss

@@ -97,6 +97,16 @@
     margin-top: 0;
   }
 
+  .sidebar-footer-container {
+    &:before {
+      background: linear-gradient(
+        to bottom,
+        rgba(var(--primary-very-low-rgb), 0),
+        rgba(var(--primary-very-low-rgb), 1)
+      );
+    }
+  }
+
   .sidebar-sections {
     display: flex;
     flex-direction: column;