Phase 4: add container-nav/cards fragment handlers, use market fragment

Events provides container-nav (calendar entries + links) and
container-cards (batch entries for blog listing) as fragments.
Day and entry routes fetch market container-nav via fragment instead
of widget registry.

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

templates/fragments/container_nav_calendars.html

@@ -0,0 +1,10 @@
+{# Calendar links nav — served as fragment from events app #}
+{% for calendar in calendars %}
+  {% set local_href=events_url('/' + post_slug + '/calendars/' + calendar.slug + '/') %}
+  <a
+    href="{{ local_href }}"
+    class="{{styles.nav_button_less_pad}}">
+    <i class="fa fa-calendar" aria-hidden="true"></i>
+    <div>{{calendar.name}}</div>
+  </a>
+{% endfor %}