fix: market URLs include post slug prefix

Nav entries template now links to /<post_slug>/<market_slug>/
matching the events app calendar URL pattern. market_url_for()
updated to accept post_slug parameter.

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

suma_browser/templates/_types/post/admin/_nav_entries.html

@@ -41,7 +41,7 @@
         {# Markets #}
         {% for m in markets %}
           <a
-            href="{{ market_url('/' + m.slug + '/') }}"
+            href="{{ market_url('/' + post.slug + '/' + m.slug + '/') }}"
             class="{{styles.nav_button_less_pad}}">
             <i class="fa fa-shopping-bag" aria-hidden="true"></i>
             <div>{{m.name}}</div>