feat(showcase): add public showcase page with per-profile/per-postcard visibility controls

app/templates/profiles.html

@@ -52,6 +52,7 @@
             <span class="profile-card-date">创建于 {{ p.created_at.strftime('%Y-%m-%d') }}</span>
             <div class="profile-card-actions">
                 <a href="/profiles/{{ p.id }}/postcards" class="btn btn-primary btn-sm">查看明信片</a>
+                <a href="/profiles/{{ p.id }}/showcase" class="btn btn-sm">展示管理</a>
                 <form method="post" action="/profiles/{{ p.id }}/delete" class="inline">
                     <button type="submit" class="btn btn-danger btn-sm" onclick="return confirm('确定删除「{{ p.nickname }}」及其所有明信片？')">删除</button>
                 </form>