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/subscribe.njk

@@ -1,4 +1,5 @@
 {# Subscribe Widget #}
+<is-land on:visible>
 <div class="widget">
   <h3 class="widget-title">Subscribe</h3>
   <div class="space-y-2">
@@ -16,3 +17,4 @@
     </a>
   </div>
 </div>
+</is-land>