feat(i18n): add English localization with per-user language preference

app/templates/showcase_manage.html

@@ -54,7 +54,7 @@
         </form>
         {% if p.showcase_enabled and pc_list %}
         <div style="margin-top:.75rem;border-top:1px solid var(--border);padding-top:.75rem">
-            <p style="font-size:.8rem;color:var(--text-muted);margin-bottom:.5rem">{{ 'profile_showcase.single_control'|t(user.language) }}：</p>
+            <p style="font-size:.8rem;color:var(--text-muted);margin-bottom:.5rem">单张明信片展示控制（关闭展示后可单独隐藏）：</p>
             {% for pc in pc_list %}
             <form method="post" action="/postcards/{{ pc.id }}/showcase-toggle" style="display:inline-flex;align-items:center;gap:.3rem;margin-right:.75rem;margin-bottom:.3rem;font-size:.8rem">
                 <span>{{ pc.card_number }}</span>
@@ -62,7 +62,7 @@
                     <input type="checkbox" name="showcase_hidden" value="1" {% if pc.showcase_hidden %}checked{% endif %} onchange="this.form.submit()">
                     <span class="toggle-slider"></span>
                 </label>
-                {% if pc.showcase_hidden %}<span style="color:var(--text-muted);font-size:.75rem">{{ 'profile_showcase.hidden'|t(user.language) }}</span>{% endif %}
+                {% if pc.showcase_hidden %}<span style="color:var(--text-muted);font-size:.75rem">隐藏</span>{% endif %}
             </form>
             {% endfor %}
         </div>