chore(frontend): harness at desktop width (768px) for two-column preview

max-w-md (448px) under-sized the panel vs its real full-width tab placement and
never triggered the desktop two-column layout. Widen to max-w-3xl; note to
resize to ~390px for the mobile (single-column) check.

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

frontend/src/dev/harness.tsx

@@ -109,7 +109,10 @@ function Case({ title, data }: { title: string; data: FundamentalResponse }) {
 }
 
 createRoot(document.getElementById('root')!).render(
-  <div className="mx-auto max-w-md space-y-8 p-4">
+  <div className="mx-auto max-w-3xl space-y-8 p-6">
+    <p className="text-[11px] uppercase tracking-widest text-gray-500">
+      Desktop width (~768px, two columns). Resize the browser to ~390px to check mobile (single column).
+    </p>
     <Case title="Full" data={full} />
     <Case title="Partial · insufficient peers" data={partial} />
     <Case title="Empty" data={empty} />