fix: harden resume and SSE replay

frontend/src/store/sessionStore.ts

@@ -16,6 +16,7 @@ interface SessionState {
   lastSystemEvent: StreamEvent | null;
   currentPhase: string | null;
   phaseError: string | null;
+  seenGateIds: Set<string>;
   // True while a Pi turn is in flight. Turned off by the backend-forwarded `agent_end`
   // system event — the only end-of-turn signal on the final workflow step, where no
   // follow-up gate arrives to release the spinner.
@@ -62,6 +63,7 @@ const empty = {
   lastSystemEvent: null,
   currentPhase: null as string | null,
   phaseError: null as string | null,
+  seenGateIds: new Set<string>(),
   agentActive: false,
 };
 
@@ -70,9 +72,11 @@ export const useSessionStore = create<SessionState>((set) => ({
   applyEvent: (e) =>
     set((st) => {
       if (e.type === "ui_request") {
+        if (st.seenGateIds.has(e.ui_request.id)) return {};
         const currentPhase = phaseOf(e.ui_request.phase, st.currentPhase);
         return {
           pendingWidget: e.ui_request,
+          seenGateIds: new Set([...st.seenGateIds, e.ui_request.id]),
           // The turn is blocked on the gate, not ended: keep the agent marked active.
           agentActive: true,
           currentPhase,