Fix admin message auto-dismiss and Category plural label

Admin messages now auto-clear after 8 seconds via the w-messages
Stimulus controller's autoClear value, preventing message pile-up.
Category model gains verbose_name_plural so Wagtail shows "Categories"
instead of "Categorys" in the snippets menu.

Closes #62

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/core/tests/test_message_handling.py

@@ -76,3 +76,21 @@ def test_wagtail_admin_template_deduplicates_consecutive_messages(client, django
     assert response.status_code == 200
     assert content.count("has been updated.") == 1
     assert content.count("has been published.") == 1
+
+
+@pytest.mark.django_db
+@override_settings(ROOT_URLCONF="apps.core.tests.test_message_handling")
+def test_admin_messages_have_auto_clear(client, django_user_model):
+    """The messages container must set auto-clear so messages dismiss themselves."""
+    admin = django_user_model.objects.create_superuser(
+        username="admin-autoclear",
+        email="admin-autoclear@example.com",
+        password="admin-pass",
+    )
+    client.force_login(admin)
+
+    response = client.get("/cms/__tests__/admin-messages/")
+    content = response.content.decode()
+
+    assert response.status_code == 200
+    assert "data-w-messages-auto-clear-value" in content