Add scroll indicator gradient to table of contents

On mobile, the TOC container clips content at max-height without
visually indicating there is more below. Adds a CSS gradient fade
at the bottom to signal scrollable content.

Fixes #1799

Author: chinar-amrutkar

common-theme/assets/styles/04-components/toc.scss

@@ -9,6 +9,19 @@
     var(--theme-spacing--4);
   overflow-y: auto;
   counter-reset: item;
+  position: relative;
+
+  // Gradient fade at bottom to indicate scrollable content (closes #1799)
+  &::after {
+    content: "";
+    position: absolute;
+    bottom: 0;
+    left: 0;
+    right: 0;
+    height: 2em;
+    background: linear-gradient(transparent, var(--theme-color--paper));
+    pointer-events: none;
+  }
 
   ol {
     color: var(--theme-color--ink);