fix(showcase): tab stay, Y-center toggle, group postcards by sent/received, hide navbar on public page, remove profiles/xx/showcase

app/templates/showcase.html

@@ -1,5 +1,6 @@
 {% extends "base.html" %}
-{% block title %}{{ profile_user.username }} 的展示页 - 明信片管理器{% endblock %}
+{% block title %}{{ profile_user.username }} 的展示页{% endblock %}
+{% block navbar %}{% endblock %}
 {% block content %}
 <div class="section-header">
     <h1>📮 {{ profile_user.username }} 的明信片</h1>