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

search.njk

@@ -19,7 +19,7 @@ pagefindIgnore: true
 </noscript>
 
 <script>
-  initPagefind("#search", { showSubResults: true });
+  initPagefind("#search", { showSubResults: true, showEmptyFilters: false });
 
   // Support ?q= query parameter and auto-focus
   window.addEventListener("DOMContentLoaded", () => {