feat(ui): show arrival date below send date for delivered postcards

app/templates/postcards.html

@@ -38,7 +38,10 @@
         <span>{{ pc.country|flag }} {{ pc.country or '-' }}</span>
         <span>→ {{ pc.recipient_name or '-' }}</span>
         <span class="badge badge-{{ pc.status }}">{{ {'sent':'已寄出','delivered':'已送达'}.get(pc.status) }}</span>
-        {% if pc.send_time %}<span class="postcard-row-date">{{ pc.send_time.strftime('%Y-%m-%d') }}寄出</span>{% endif %}
+        <span class="postcard-row-date">
+            {% if pc.send_time %}{{ pc.send_time.strftime('%Y-%m-%d') }}寄出{% endif %}
+            {% if pc.arrival_time %}<br>{{ pc.arrival_time.strftime('%Y-%m-%d') }}送达{% endif %}
+        </span>
     </div>
     <div class="postcard-row-action">
         {% if pc.status == 'sent' %}