fix(spacing): balance pagination-nav padding and add footer margin

Reduce excessive bottom padding on pagination-nav and distribute
it more evenly between top and bottom. Add matching margin-bottom
to doc footer for visual balance.

Changes:
- Change pagination-nav from 50vh bottom-only to 2rem top + 20vh bottom
- Add 3rem margin-bottom to .theme-doc-footer (matching margin-top)

This provides enough space for the last TOC item to activate while
maintaining better visual balance and less excessive whitespace.

Author: claude

src/css/custom.css

@@ -534,5 +534,11 @@ article {
 
 /* Add padding to pagination nav to allow last TOC item to scroll into view */
 .pagination-nav {
-  padding-bottom: 50vh;
+  padding-top: 2rem;
+  padding-bottom: 20vh;
+}
+
+/* Add bottom margin to doc footer matching top margin */
+.theme-doc-footer {
+  margin-bottom: 3rem;
 }