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/blog/tests/test_admin_experience.py

@@ -386,6 +386,31 @@ def test_article_save_autogenerates_summary_when_missing(article_index):
     assert article.summary == "This should become the summary text."
 
 
+@pytest.mark.django_db
+def test_category_verbose_name_plural():
+    """Category Meta should define verbose_name_plural as 'categories'."""
+    assert Category._meta.verbose_name_plural == "categories"
+
+
+@pytest.mark.django_db
+@override_settings(ALLOWED_HOSTS=["testserver", "localhost", "127.0.0.1"])
+def test_snippet_category_listing_shows_categories(client, django_user_model):
+    """Categories created in the database should appear in the Snippets listing."""
+    Category.objects.create(name="Reviews", slug="reviews")
+    Category.objects.create(name="Tutorials", slug="tutorials")
+
+    admin = django_user_model.objects.create_superuser(
+        username="admin-cat", email="admin-cat@example.com", password="admin-pass"
+    )
+    client.force_login(admin)
+    response = client.get("/cms/snippets/blog/category/")
+    content = response.content.decode()
+
+    assert response.status_code == 200
+    assert "Reviews" in content
+    assert "Tutorials" in content
+
+
 @pytest.mark.django_db
 def test_article_page_omits_admin_messages_on_frontend(article_page, rf):
     """Frontend templates should not render admin session messages."""