feat(FN-1152): add retry flows for failed AI planning sessions

- Keep errored AI sessions retryable in the session store and add backend retry handlers for planning, subtask breakdown, and mission interview flows
- Add retry API routes and dashboard API client helpers for planning, subtask, and mission interview session retries
- Update PlanningModeModal, SubtaskBreakdownModal, MissionInterviewModal, and background session handling to show error states with retry/cancel UX
- Expand unit and integration tests across store, services, routes, and modal components to cover retry success and failure paths

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

@@ -4,6 +4,7 @@ import { MissionInterviewModal } from "./MissionInterviewModal";
 
 const mockStartMissionInterview = vi.fn();
 const mockRespondToMissionInterview = vi.fn();
+const mockRetryMissionInterviewSession = vi.fn();
 const mockCancelMissionInterview = vi.fn();
 const mockCreateMissionFromInterview = vi.fn();
 const mockConnectMissionInterviewStream = vi.fn();
@@ -13,6 +14,7 @@ const mockParseConversationHistory = vi.fn();
 vi.mock("../api", () => ({
   startMissionInterview: (...args: any[]) => mockStartMissionInterview(...args),
   respondToMissionInterview: (...args: any[]) => mockRespondToMissionInterview(...args),
+  retryMissionInterviewSession: (...args: any[]) => mockRetryMissionInterviewSession(...args),
   cancelMissionInterview: (...args: any[]) => mockCancelMissionInterview(...args),
   createMissionFromInterview: (...args: any[]) => mockCreateMissionFromInterview(...args),
   connectMissionInterviewStream: (...args: any[]) => mockConnectMissionInterviewStream(...args),
@@ -45,6 +47,7 @@ describe("MissionInterviewModal", () => {
     streamHandlers = undefined;
 
     mockStartMissionInterview.mockResolvedValue({ sessionId: "mission-session-1" });
+    mockRetryMissionInterviewSession.mockResolvedValue({ success: true, sessionId: "mission-session-1" });
     mockFetchAiSession.mockResolvedValue(null);
     mockParseConversationHistory.mockImplementation((raw: string) => {
       if (!raw) return [];
@@ -135,4 +138,62 @@ describe("MissionInterviewModal", () => {
     expect(screen.getByText("Reconnecting…")).toBeInTheDocument();
     expect(screen.getByText("Analyzing mission goals...")).toBeInTheDocument();
   });
+
+  it("shows error panel with retry action when stream fails", 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.onError?.("Temporary outage");
+    });
+
+    expect(await screen.findByText("Temporary outage")).toBeInTheDocument();
+    expect(screen.getByRole("button", { name: "Retry" })).toBeInTheDocument();
+  });
+
+  it("retries interview session from error view", async () => {
+    let attempt = 0;
+    mockConnectMissionInterviewStream.mockImplementation((_sessionId, _projectId, handlers) => {
+      streamHandlers = handlers;
+      attempt += 1;
+      if (attempt === 1) {
+        setTimeout(() => handlers.onError?.("Try again"), 10);
+      } else {
+        setTimeout(() => handlers.onQuestion?.(SAMPLE_QUESTION), 10);
+      }
+      return {
+        close: vi.fn(),
+        isConnected: vi.fn().mockReturnValue(true),
+      };
+    });
+
+    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(screen.getByText("Try again")).toBeInTheDocument();
+    });
+
+    fireEvent.click(screen.getByRole("button", { name: "Retry" }));
+
+    await waitFor(() => {
+      expect(mockRetryMissionInterviewSession).toHaveBeenCalledWith("mission-session-1", undefined);
+    });
+    await waitFor(() => {
+      expect(screen.getByText("What is the target scope?")).toBeInTheDocument();
+    });
+    expect(mockConnectMissionInterviewStream).toHaveBeenCalledTimes(2);
+  });
 });