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/MarkdownView.tsx

@@ -42,7 +42,7 @@ const components: Components = {
 
 export function MarkdownView({ source }: { source: string }) {
   return (
-    <div className="markdown-view overflow-auto max-h-[60vh] prose prose-sm dark:prose-invert p-3">
+    <div className="markdown-view thot-prose">
       <ReactMarkdown remarkPlugins={[remarkGfm]} components={components}>
         {source}
       </ReactMarkdown>