Archived chats no longer show inside folders

The unfiled list has filtered archived chats since archiving existed
(api/pages.py). The folder branch went through the ORM relationship, which
filters nothing, so an archived chat kept appearing as long as it was
filed -- and the "Empty" check read the same unfiltered list, so a folder
holding only archived chats would have claimed to be empty while listing
them.

Fixed on the model rather than in the template, as `Folder.visible_chats`.
The loop and the empty check now cannot disagree, because there is one
list and the template binds it once. Ordering matches the unfiled list:
pinned first, then most recently touched.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/web/templates/partials/_folder.html

@@ -30,17 +30,22 @@
   </div>
 
   <div class="folder__contents" x-show="open" x-cloak>
+    {# Bound once: the loop and the "Empty" check must be looking at the same
+       list, or a folder holding only archived chats claims to be empty while
+       showing them. #}
+    {% set listed = folder.visible_chats %}
+
     {% for child in folder.children %}
       {% with folder = child %}
         {% include "partials/_folder.html" %}
       {% endwith %}
     {% endfor %}
 
-    {% for chat_item in folder.chats %}
+    {% for chat_item in listed %}
       {% include "partials/_chat_link.html" %}
     {% endfor %}
 
-    {% if not folder.children and not folder.chats %}
+    {% if not folder.children and not listed %}
     <p class="nav-empty">Empty</p>
     {% endif %}
   </div>