feat: add link to /ai/ page in AI usage disclosure

The "Learn more about AI usage on this site" link appears when
the AI usage label is clicked/tapped on posts, providing context
about the site's AI transparency policy.

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

_includes/layouts/page.njk

@@ -107,6 +107,7 @@ withSidebar: true
     {% if aiDescription %}
     <p class="mt-2 text-xs text-surface-500 dark:text-surface-400">{{ aiDescription }}</p>
     {% endif %}
+    <p class="mt-2 text-xs"><a href="/ai/" class="text-accent-600 dark:text-accent-400 hover:underline">Learn more about AI usage on this site &rarr;</a></p>
   </aside>
   {% endif %}
 

_includes/layouts/post.njk

@@ -85,6 +85,7 @@ withBlogSidebar: true
     {% if aiDescription %}
     <p class="mt-1 pl-5">{{ aiDescription }}</p>
     {% endif %}
+    <p class="mt-1 pl-5"><a href="/ai/" class="hover:text-accent-600 dark:hover:text-accent-400 underline">Learn more about AI usage on this site</a></p>
   </details>
 
   {# Pending syndication targets (for services like IndieNews that require u-syndication before webmention) #}