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/db/models/chat.py

@@ -45,6 +45,24 @@ class Folder(UUIDPrimaryKey, Timestamps, Base):
     parent: Mapped[Folder | None] = relationship(back_populates="children", remote_side="Folder.id")
     chats: Mapped[list[Chat]] = relationship(back_populates="folder")
 
+    @property
+    def visible_chats(self) -> list[Chat]:
+        """The chats in this folder that belong in the sidebar.
+
+        The relationship itself stays unfiltered -- back-population needs every
+        row -- so the listing rule lives here rather than in the template, where
+        the loop and the "Empty" check would have to agree by hand and already
+        did not: archived chats have been showing inside folders since folders
+        existed. The unfiled list has always filtered them (api/pages.py); the
+        folder branch went through the relationship and filtered nothing.
+
+        Ordered like the unfiled list: pinned first, then most recently touched.
+        """
+        kept = [chat for chat in self.chats if not chat.archived]
+        kept.sort(key=lambda chat: chat.updated_at, reverse=True)
+        kept.sort(key=lambda chat: not chat.pinned)
+        return kept
+
     def __repr__(self) -> str:
         return f"<Folder {self.name}>"