feat: blog filter nav, interactions pagination, note unfurl, pagefind improvements

- Replace broken client-side type filter on /blog/ with navigation
  pill links to dedicated collection pages (with post counts)
- Replace Load More with proper prev/next/page-number pagination
  on Interactions inbound tab (20 per page, filter resets page)
- Add auto-unfurl transform for standalone external links in notes
- Exclude Digest and Categories pages from Pagefind search index
- Add Pagefind search filters for post type, year, and category
- Add Pagefind filter metadata to page.njk layout

Confab-Link: http://localhost:8080/sessions/956f4251-b4a9-4bc9-b214-53402ad1fe63

_includes/layouts/page.njk

@@ -133,4 +133,18 @@ withSidebar: true
   {# Hidden metadata for microformats #}
   <a class="u-url hidden" href="{{ page.url }}"></a>
   <data class="p-author h-card hidden" value="{{ site.author.name }}"></data>
+
+  {# Pagefind filter metadata #}
+  <div hidden>
+    <span data-pagefind-filter="type">Page</span>
+    {% if category %}
+      {% if category is string %}
+      <span data-pagefind-filter="category">{{ category }}</span>
+      {% else %}
+      {% for cat in category %}
+      <span data-pagefind-filter="category">{{ cat }}</span>
+      {% endfor %}
+      {% endif %}
+    {% endif %}
+  </div>
 </article>