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/SubtaskBreakdownModal.test.tsx

@@ -3,12 +3,14 @@ import { act, render, screen, fireEvent, waitFor } from "@testing-library/react"
 import { SubtaskBreakdownModal } from "./SubtaskBreakdownModal";
 
 const mockStartSubtaskBreakdown = vi.fn();
+const mockRetrySubtaskSession = vi.fn();
 const mockConnectSubtaskStream = vi.fn();
 const mockCreateTasksFromBreakdown = vi.fn();
 const mockCancelSubtaskBreakdown = vi.fn();
 
 vi.mock("../api", () => ({
   startSubtaskBreakdown: (...args: any[]) => mockStartSubtaskBreakdown(...args),
+  retrySubtaskSession: (...args: any[]) => mockRetrySubtaskSession(...args),
   connectSubtaskStream: (...args: any[]) => mockConnectSubtaskStream(...args),
   createTasksFromBreakdown: (...args: any[]) => mockCreateTasksFromBreakdown(...args),
   cancelSubtaskBreakdown: (...args: any[]) => mockCancelSubtaskBreakdown(...args),
@@ -40,6 +42,7 @@ describe("SubtaskBreakdownModal", () => {
     vi.clearAllMocks();
     streamHandlers = undefined;
     mockStartSubtaskBreakdown.mockResolvedValue({ sessionId: "session-123" });
+    mockRetrySubtaskSession.mockResolvedValue({ success: true, sessionId: "session-123" });
     mockConnectSubtaskStream.mockImplementation((_sessionId, _projectId, handlers) => {
       streamHandlers = handlers;
       return { close: vi.fn(), isConnected: () => true };
@@ -483,6 +486,20 @@ describe("SubtaskBreakdownModal", () => {
       expect(await screen.findByText("Something went wrong")).toBeInTheDocument();
     });
 
+    it("retries after an error and reconnects stream", async () => {
+      renderModal();
+      await waitFor(() => expect(streamHandlers).toBeDefined());
+      streamHandlers.onError("Something went wrong");
+
+      const retryButton = await screen.findByRole("button", { name: "Retry" });
+      fireEvent.click(retryButton);
+
+      await waitFor(() => {
+        expect(mockRetrySubtaskSession).toHaveBeenCalledWith("session-123", undefined);
+      });
+      expect(mockConnectSubtaskStream).toHaveBeenCalledTimes(2);
+    });
+
     it("shows Stream error fallback when receiving empty error", async () => {
       renderModal();
       await waitFor(() => expect(streamHandlers).toBeDefined());