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>

tests/test_chat.py

@@ -314,6 +314,28 @@ def test_deleting_a_folder_keeps_the_chats_inside_it(client: TestClient, db, reg
     assert chat.folder_id is None
 
 
+def test_an_archived_chat_inside_a_folder_is_not_listed(
+    client: TestClient, db, registered, make_chat
+):
+    """Regression: the unfiled list has always filtered archived chats, but the
+    folder branch went through the ORM relationship and filtered nothing, so an
+    archived chat kept showing as long as it was filed."""
+    _add_connection(db)
+    client.post("/api/folders", data={"name": "Quests"})
+    folder = db.scalar(select(Folder))
+
+    chat_id = make_chat()
+    client.patch(f"/api/chats/{chat_id}", data={"folder_id": folder.id, "title": "Mount Doom"})
+    chat = db.get(Chat, chat_id)
+    chat.archived = True
+    db.commit()
+
+    page = client.get("/chat").text
+    assert "Mount Doom" not in page
+    # And the folder must say so, rather than claiming to hold something.
+    assert "Empty" in page
+
+
 def test_a_folder_cannot_be_moved_inside_itself(client: TestClient, db, registered):
     client.post("/api/folders", data={"name": "Outer"})
     folder = db.scalar(select(Folder))