feat: wrap all widgets and comments in is-land on:visible

Add <is-land on:visible> lazy-loading wrapper to every widget template
and the comments section for consistent deferred rendering. Widgets
that already had it (social-activity, github-repos, blogroll, feedland,
webmentions) are unchanged. Also wraps inline search and custom-html
widgets in all sidebar container files.

_includes/components/widgets/toc.njk

@@ -1,5 +1,6 @@
 {# Table of Contents Widget (for articles with headings) #}
 {% if toc and toc.length %}
+<is-land on:visible>
 <div class="widget">
   <h3 class="widget-title">Contents</h3>
   <nav class="toc">
@@ -14,4 +15,5 @@
     </ul>
   </nav>
 </div>
+</is-land>
 {% endif %}