feat(widgets): wire PostsNavs TOC shell to page-template scrollspy

Match built-in single sidebar .posts-nav-box.toc-widget/.toc-list so the
sidebar widget is filled (≥3 headings), supports multiple TOC shells, and
removes empty chrome.

assets/css/frontend/single-post/related-toc.css

@@ -364,6 +364,10 @@ html.dark .article-end-separator span {
     color: var(--color-primary);
 }
 
+.toc-list .toc-h1 a {
+    font-weight: 700;
+}
+
 .toc-list .toc-h3 a {
     padding-left: 12px;
 }
@@ -372,6 +376,30 @@ html.dark .article-end-separator span {
     padding-left: 24px;
 }
 
+/* WidgetPostsNavs + built-in shell share .posts-nav-box.toc-widget */
+.widget .posts-nav-box.toc-widget,
+.widget_aksrt_posts_navs .posts-nav-box.toc-widget {
+    margin: 0;
+    box-shadow: none;
+    background: transparent;
+    padding: 0;
+    border-radius: 0;
+}
+
+.widget .posts-nav-box.toc-widget > h3,
+.widget .posts-nav-box.toc-widget .toc-widget__title,
+.widget_aksrt_posts_navs .posts-nav-box.toc-widget > h3 {
+    margin: 0 0 10px;
+    font-size: 15px;
+    font-weight: 700;
+}
+
+body.single .sidebar .posts-nav-box.toc-widget.is-affix,
+body.single .sidebar .posts-nav-box.toc-widget[data-affix="true"] {
+    position: sticky;
+    top: calc(var(--nav-height, 60px) + 16px);
+}
+
 .toc-float-btn {
     position: fixed;
     right: 18px;