Fix empty-category nav and route behavior

Use category-state-driven queries for nav and category listing routes, and add regression tests for empty but valid categories.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

apps/core/tests/test_tags.py

@@ -37,3 +37,14 @@ def test_categories_nav_tag_renders_category_link(client, home_page):
     resp = client.get("/")
     assert resp.status_code == 200
     assert "/articles/category/reviews/" in resp.content.decode()
+
+
+@pytest.mark.django_db
+def test_categories_nav_tag_includes_empty_nav_category(client, home_page):
+    index = ArticleIndexPage(title="Articles", slug="articles")
+    home_page.add_child(instance=index)
+    Category.objects.create(name="Benchmarks", slug="benchmarks", show_in_nav=True)
+
+    resp = client.get("/")
+    assert resp.status_code == 200
+    assert "/articles/category/benchmarks/" in resp.content.decode()