fix: make TOC gradient sticky to viewport bottom

Use position: sticky instead of absolute so the gradient stays fixed
at the bottom of the visible scroll area as the user scrolls, rather
than staying pinned to the bottom of the scrollable content.

Addresses review feedback from illicitonion.

Author: chinar-amrutkar

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

@@ -9,18 +9,17 @@
     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;
+    position: sticky;
     bottom: 0;
-    left: 0;
-    right: 0;
+    display: block;
     height: 2em;
+    margin-top: -2em;
     background: linear-gradient(transparent, var(--theme-color--paper));
     pointer-events: none;
+    flex-shrink: 0;
   }
 
   ol {