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/store/sessionStore.ts

@@ -16,6 +16,7 @@ interface SessionState {
   clearPending: () => void;
   resetSession: () => void;
   setPhase: (phase: string | null) => void;
+  pushToast: (toast: { level: string; text: string }) => void;
 }
 
 const empty = {
@@ -30,7 +31,13 @@ export const useSessionStore = create<SessionState>((set) => ({
   ...empty,
   applyEvent: (e) =>
     set((st) => {
-      if (e.type === "ui_request") return { pendingWidget: e.ui_request };
+      if (e.type === "ui_request")
+        return {
+          pendingWidget: e.ui_request,
+          currentPhase: e.ui_request.phase
+            ? e.ui_request.phase.split("_")[0]
+            : st.currentPhase,
+        };
       if (e.type === "text_delta") {
         const t = [...st.transcript];
         const last = t.at(-1);
@@ -45,4 +52,5 @@ export const useSessionStore = create<SessionState>((set) => ({
   clearPending: () => set({ pendingWidget: null }),
   resetSession: () => set({ ...empty }),
   setPhase: (phase) => set({ currentPhase: phase }),
+  pushToast: (toast) => set((st) => ({ toasts: [...st.toasts, toast] })),
 }));