feat(ui): show country flag emoji next to country codes

app/templates/postcard_detail.html

@@ -44,7 +44,7 @@
             <dt>状态</dt><dd>{{ {'sent':'已寄出','delivered':'已送达','received':'已收到'}.get(postcard.status, postcard.status) }}</dd>
             {% if postcard.status in ('sent','delivered') %}
             <dt>收件人</dt><dd>{{ postcard.recipient_name or '-' }}</dd>
-            <dt>国家/地区</dt><dd>{{ postcard.country or '-' }}</dd>
+            <dt>国家/地区</dt><dd>{{ postcard.country|flag }} {{ postcard.country or '-' }}</dd>
             <dt>寄出时间</dt><dd>{{ postcard.send_time.strftime('%Y-%m-%d') if postcard.send_time else '-' }}</dd>
             <dt>到达时间</dt><dd>{{ postcard.arrival_time.strftime('%Y-%m-%d') if postcard.arrival_time else '-' }}</dd>
             {% else %}