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/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");
+  }
+});