fix: use trip-relative URL for entry back-link, add active_trip sync comment

- entry.html.twig: replace hardcoded /tracker href with page.parent().url
- post-form.md: add comment to keep pageconfig.parent in sync with active_trip in site.yaml

pages/02.post/post-form.md

@@ -4,6 +4,7 @@ template: post-form
 access:
     site.login: true
 
+# Keep in sync with active_trip in user/config/site.yaml
 pageconfig:
     parent: '/trips/japan-korea-2026/dailies'
     slug_field: 'date,title'

themes/intotheeast/templates/entry.html.twig

@@ -121,7 +121,7 @@
     {% endif %}
 
     <footer class="entry-footer">
-        <a href="{{ base_url_absolute }}/tracker">← Back to journal</a>
+        <a href="{{ page.parent().url }}">← Back to journal</a>
     </footer>
 </article>
 {% endblock %}