feat: dynamic magazine category filter from published articles

Add articleCategories GraphQL query returning distinct categories from
published articles; magazine page fetches them at load time and renders
only categories that actually have content.

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

packages/frontend/src/lib/services.ts

@@ -298,6 +298,23 @@ export async function getArticles(
   });
 }
 
+const ARTICLE_CATEGORIES_QUERY = gql`
+  query GetArticleCategories {
+    articleCategories
+  }
+`;
+
+export async function getArticleCategories(
+  fetchFn?: typeof globalThis.fetch,
+): Promise<string[]> {
+  return loggedApiCall("getArticleCategories", async () => {
+    const data = await getGraphQLClient(fetchFn).request<{ articleCategories: string[] }>(
+      ARTICLE_CATEGORIES_QUERY,
+    );
+    return data.articleCategories;
+  });
+}
+
 const ARTICLE_BY_SLUG_QUERY = gql`
   query GetArticleBySlug($slug: String!) {
     article(slug: $slug) {