feat(ui): add back button left of card number on detail page

app/static/style.css

@@ -215,6 +215,9 @@ code { background: var(--bg); padding: .15rem .4rem; border-radius: 4px; font-si
 .postcard-row-action { flex-shrink: 0; }
 .postcard-row-date { font-size: .78rem; color: var(--text-muted); white-space: nowrap; }
 .postcard-row-detail { display: flex; flex-direction: column; gap: 0; font-size: .85rem; color: var(--text-muted); }
+.detail-title-row { display: flex; align-items: center; gap: .75rem; }
+.detail-title-row h1 { margin: 0; }
+.btn-back { padding: .35rem .6rem; font-size: 1rem; text-decoration: none; }
 
 /* Filter */
 .filter-bar { margin-bottom: 1.25rem; }

app/templates/postcard_detail.html

@@ -2,7 +2,10 @@
 {% block title %}明信片详情 - 明信片管理器{% endblock %}
 {% block content %}
 <div class="section-header">
+    <div class="detail-title-row">
+        <a href="/profiles/{{ postcard.profile_id }}/postcards" class="btn btn-back">←</a>
         <h1>{{ postcard.card_number }}</h1>
+    </div>
     <div>
         <a href="/postcards/{{ postcard.id }}/edit" class="btn">编辑</a>
         {% if postcard.status == 'sent' %}