fix(ui): shorten mark-delivered button text to 已送达

app/templates/postcard_detail.html

@@ -10,7 +10,7 @@
         <a href="/postcards/{{ postcard.id }}/edit" class="btn">编辑</a>
         {% if postcard.status == 'sent' %}
         <form method="post" action="/postcards/{{ postcard.id }}/mark-delivered" class="inline">
-            <button type="submit" class="btn btn-primary" onclick="return confirm('确认已送达？')">标记已送达</button>
+            <button type="submit" class="btn btn-primary" onclick="return confirm('确认已送达？')">已送达</button>
         </form>
         {% endif %}
         <form method="post" action="/postcards/{{ postcard.id }}/delete" class="inline">