feat(viewer): artifact modal renders structured text, never raw JSON

ArtifactView's fallback (StructuredValue) turns any object/array into labelled
sections, bullet lists and paragraphs (strings as markdown). Verified against the
real F1 gate (kind:"text", {recap, chiarimenti_risolti[], ...}). MarkdownView
uses the real .thot-prose class (Tailwind Typography is not installed).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/viewers/ArtifactView.test.tsx

@@ -26,9 +26,23 @@ test("question renders markdown headings", () => {
   expect(screen.getByText("Domanda")).toBeInTheDocument();
 });
 
-test("unknown kind falls back to formatted JSON", () => {
-  render(<ArtifactView artifact={{ kind: "mystery", data: { a: 1 } }} />);
-  expect(screen.getByText(/"a": 1/)).toBeInTheDocument();
+test("unknown object kind renders as formatted sections, never raw JSON", () => {
+  render(
+    <ArtifactView
+      artifact={{
+        kind: "phase",
+        data: {
+          recap: "Domanda: lista pazienti.",
+          chiarimenti_risolti: ["finestra_temporale — 5 anni", "tipo_evento — solo impianto"],
+        },
+      }}
+    />,
+  );
+  // Humanized section labels, values as text/bullets — and NO raw JSON braces.
+  expect(screen.getByText("Recap")).toBeInTheDocument();
+  expect(screen.getByText("Chiarimenti risolti")).toBeInTheDocument();
+  expect(screen.getByText("finestra_temporale — 5 anni")).toBeInTheDocument();
+  expect(screen.queryByText(/"recap"/)).not.toBeInTheDocument();
 });
 
 test("schema_linking renders the schema viewer", async () => {