fix(frontend): wire currentPhase to WorkflowBar (normalized) + WidgetHost error handling

- sessionStore.applyEvent: on ui_request set currentPhase normalized from the
  descriptor's phase to its short id (e.g. "F4_schema_linking" -> "F4"), so
  WorkflowBar actually highlights the active phase; falls back to the existing
  phase when the descriptor has none. setPhase kept for resume/getSession.
- Add pushToast store action; WidgetHost wraps postResponse in try/catch,
  pushes an error toast and keeps the widget pending on failure (clearPending
  only on success) so the user can retry.
- Tests: store currentPhase normalization + no-phase passthrough + pushToast;
  new WorkflowBar.test.tsx asserting the matching phase is highlighted.

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

frontend/src/shell/WidgetHost.tsx

@@ -6,10 +6,25 @@ import type { UiResponse } from "../api/types";
 export function WidgetHost({ sessionId }: { sessionId: string | null }) {
   const pending = useSessionStore((s) => s.pendingWidget);
   const clearPending = useSessionStore((s) => s.clearPending);
+  const pushToast = useSessionStore((s) => s.pushToast);
   if (!pending) return null;
   const Renderer = resolve(pending.widget);
   const onRespond = async (r: UiResponse) => {
-    if (sessionId) await postResponse(sessionId, r);
+    if (sessionId) {
+      try {
+        await postResponse(sessionId, r);
+      } catch (err) {
+        // Surface feedback and keep the widget pending so the user can retry.
+        pushToast({
+          level: "error",
+          text:
+            err instanceof Error
+              ? `Invio risposta fallito: ${err.message}`
+              : "Invio risposta fallito.",
+        });
+        return;
+      }
+    }
     clearPending();
   };
   return <Renderer descriptor={pending} onRespond={onRespond} />;