fix(frontend): expose completed result state

frontend/src/App.jsx

@@ -116,7 +116,12 @@ export function App() {
           {needsSignIn ? <p className="hint">The page is public, but platform auth is required for account trial calls and durable history.</p> : null}
         </form>
 
-        <section className="result-card" data-testid="agent-result" aria-live="polite">
+        <section
+          className="result-card"
+          data-testid="agent-result"
+          data-state={running ? "loading" : error ? "error" : result ? "success" : "idle"}
+          aria-live="polite"
+        >
           {!result ? (
             <div className="empty-state">
               <p className="eyebrow">Awaiting run</p>