feat(FN-1145): harden dashboard SSE streams with replay and reconnect

- Add a shared SSE event buffer utility and wire it into planning, subtask, mission interview, and task stream routes
- Support Last-Event-ID replay semantics and robust event serialization so clients can recover missed stream events
- Add a resilient client-side reconnect wrapper and surface reconnecting state in planning, subtask breakdown, and mission interview modals
- Expand test coverage across API routes, SSE buffering behavior, reconnect handling, and mission/planning stream flows

packages/dashboard/app/components/MissionInterviewModal.test.tsx

@@ -0,0 +1,127 @@
+import { act, fireEvent, render, screen, waitFor } from "@testing-library/react";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { MissionInterviewModal } from "./MissionInterviewModal";
+
+const mockStartMissionInterview = vi.fn();
+const mockRespondToMissionInterview = vi.fn();
+const mockCancelMissionInterview = vi.fn();
+const mockCreateMissionFromInterview = vi.fn();
+const mockConnectMissionInterviewStream = vi.fn();
+const mockFetchAiSession = vi.fn();
+
+vi.mock("../api", () => ({
+  startMissionInterview: (...args: any[]) => mockStartMissionInterview(...args),
+  respondToMissionInterview: (...args: any[]) => mockRespondToMissionInterview(...args),
+  cancelMissionInterview: (...args: any[]) => mockCancelMissionInterview(...args),
+  createMissionFromInterview: (...args: any[]) => mockCreateMissionFromInterview(...args),
+  connectMissionInterviewStream: (...args: any[]) => mockConnectMissionInterviewStream(...args),
+  fetchAiSession: (...args: any[]) => mockFetchAiSession(...args),
+}));
+
+vi.mock("../hooks/modalPersistence", () => ({
+  saveMissionGoal: vi.fn(),
+  getMissionGoal: vi.fn(() => ""),
+  clearMissionGoal: vi.fn(),
+}));
+
+const SAMPLE_QUESTION = {
+  id: "scope",
+  type: "single_select" as const,
+  question: "What is the target scope?",
+  description: "Pick the size for this mission.",
+  options: [
+    { id: "mvp", label: "MVP" },
+    { id: "full", label: "Full" },
+  ],
+};
+
+describe("MissionInterviewModal", () => {
+  let streamHandlers: any;
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    streamHandlers = undefined;
+
+    mockStartMissionInterview.mockResolvedValue({ sessionId: "mission-session-1" });
+    mockFetchAiSession.mockResolvedValue(null);
+    mockConnectMissionInterviewStream.mockImplementation((_sessionId, _projectId, handlers) => {
+      streamHandlers = handlers;
+      return {
+        close: vi.fn(),
+        isConnected: vi.fn().mockReturnValue(true),
+      };
+    });
+  });
+
+  function renderModal() {
+    return render(
+      <MissionInterviewModal
+        isOpen={true}
+        onClose={vi.fn()}
+        onMissionCreated={vi.fn()}
+      />,
+    );
+  }
+
+  it("shows reconnecting indicator without clearing current question", async () => {
+    renderModal();
+
+    fireEvent.change(screen.getByLabelText("What do you want to build?"), {
+      target: { value: "Build a mission planning workflow" },
+    });
+    fireEvent.click(screen.getByText("Start Interview"));
+
+    await waitFor(() => {
+      expect(mockStartMissionInterview).toHaveBeenCalledWith("Build a mission planning workflow", undefined);
+      expect(streamHandlers).toBeDefined();
+    });
+
+    act(() => {
+      streamHandlers.onQuestion?.(SAMPLE_QUESTION);
+    });
+
+    expect(await screen.findByText("What is the target scope?")).toBeInTheDocument();
+
+    act(() => {
+      streamHandlers.onConnectionStateChange?.("reconnecting");
+    });
+
+    expect(screen.getByText("Reconnecting…")).toBeInTheDocument();
+    expect(screen.getByText("What is the target scope?")).toBeInTheDocument();
+
+    act(() => {
+      streamHandlers.onConnectionStateChange?.("connected");
+    });
+
+    await waitFor(() => {
+      expect(screen.queryByText("Reconnecting…")).not.toBeInTheDocument();
+    });
+    expect(screen.getByText("What is the target scope?")).toBeInTheDocument();
+  });
+
+  it("preserves streaming thinking output while reconnecting", async () => {
+    renderModal();
+
+    fireEvent.change(screen.getByLabelText("What do you want to build?"), {
+      target: { value: "Build a mission planning workflow" },
+    });
+    fireEvent.click(screen.getByText("Start Interview"));
+
+    await waitFor(() => {
+      expect(streamHandlers).toBeDefined();
+    });
+
+    act(() => {
+      streamHandlers.onThinking?.("Analyzing mission goals...");
+    });
+
+    expect(await screen.findByText("Analyzing mission goals...")).toBeInTheDocument();
+
+    act(() => {
+      streamHandlers.onConnectionStateChange?.("reconnecting");
+    });
+
+    expect(screen.getByText("Reconnecting…")).toBeInTheDocument();
+    expect(screen.getByText("Analyzing mission goals...")).toBeInTheDocument();
+  });
+});