feat(ui): show send/receive date in postcard list

app/templates/postcards.html

@@ -38,6 +38,7 @@
         <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 %}
     </div>
     <div class="postcard-row-action">
         {% if pc.status == 'sent' %}
@@ -67,6 +68,7 @@
     <div class="postcard-row-info">
         <strong>{{ pc.card_number }}</strong>
         <span>← {{ pc.sender_name or '-' }}</span>
+        {% if pc.receive_time %}<span class="postcard-row-date">{{ pc.receive_time.strftime('%Y-%m-%d') }}收到</span>{% endif %}
     </div>
 </a>
 {% endfor %}