refactor(post): replace custom Interactions with Webmentions section, renamed to Interactions

Remove post-interactions.njk include and rename the Webmentions heading to "Interactions".

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

_includes/components/webmentions.njk

@@ -17,7 +17,7 @@
 {% if mentions.length %}
 <section class="webmentions mt-8 pt-8 border-t border-surface-200 dark:border-surface-700" id="webmentions">
   <h2 class="text-xl font-bold text-surface-900 dark:text-surface-100 mb-6">
-    Webmentions ({{ mentions.length }})
+    Interactions ({{ mentions.length }})
   </h2>
 
   {# Likes #}

_includes/layouts/post.njk

@@ -315,8 +315,6 @@ withBlogSidebar: true
   </template>
 </article>
 
-{# Interactions — inbound webmentions for this post, hidden when none #}
-{% include "components/post-interactions.njk" %}
 
 {# Comments section #}
 {% include "components/comments.njk" %}