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} />;

frontend/src/shell/WorkflowBar.test.tsx

@@ -0,0 +1,22 @@
+// frontend/src/shell/WorkflowBar.test.tsx
+import { render, screen } from "@testing-library/react";
+import { WorkflowBar } from "./WorkflowBar";
+import { useSessionStore } from "../store/sessionStore";
+
+beforeEach(() => useSessionStore.getState().resetSession());
+
+test("highlights the phase matching currentPhase", () => {
+  useSessionStore.getState().setPhase("F4");
+  render(<WorkflowBar />);
+  expect(screen.getByTestId("phase-F4")).toHaveAttribute("data-active", "true");
+  // Other phases are not highlighted.
+  expect(screen.getByTestId("phase-F1")).toHaveAttribute("data-active", "false");
+  expect(screen.getByTestId("phase-F8")).toHaveAttribute("data-active", "false");
+});
+
+test("no phase highlighted when currentPhase is null", () => {
+  render(<WorkflowBar />);
+  for (const id of ["F1", "F2", "F3", "F4", "F5", "F6", "F7", "F8"]) {
+    expect(screen.getByTestId(`phase-${id}`)).toHaveAttribute("data-active", "false");
+  }
+});