feat: add market links to post nav entries template

Markets now appear alongside calendars in the post nav bar,
linking to the market app via market_url().

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

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

@@ -37,6 +37,16 @@
             <div>{{calendar.name}}</div>
           </a>
         {% endfor %}
+
+        {# Markets #}
+        {% for m in markets %}
+          <a
+            href="{{ market_url('/' + m.slug + '/') }}"
+            class="{{styles.nav_button_less_pad}}">
+            <i class="fa fa-shopping-bag" aria-hidden="true"></i>
+            <div>{{m.name}}</div>
+          </a>
+        {% endfor %}
       </div>
     </div>