feat(FN-5204): add test mode activation across settings, engine, and dashboard

- Add core settings schema/types support for testMode with model-resolution override handling
- Enforce engine session lane overrides in test mode with targeted helper coverage
- Add dashboard settings toggle plus persistent test-mode banner and related component tests
- Update settings documentation and parity/roundtrip tests for the new test mode behavior

packages/dashboard/app/components/__tests__/SettingsModal.testMode.test.tsx

@@ -0,0 +1,87 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { render, screen, waitFor } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { SettingsModal } from "../SettingsModal";
+
+const mockFetchSettings = vi.fn();
+const mockFetchSettingsByScope = vi.fn();
+
+vi.mock("../../api", async (importOriginal) => {
+  const { createDashboardApiMock } = await import("../../test/mockApi");
+  return createDashboardApiMock(() => importOriginal<typeof import("../../api")>(), {
+    fetchSettings: (...args: unknown[]) => mockFetchSettings(...args),
+    fetchSettingsByScope: (...args: unknown[]) => mockFetchSettingsByScope(...args),
+  });
+});
+
+vi.mock("../../hooks/useMemoryBackendStatus", () => ({
+  useMemoryBackendStatus: () => ({ status: null, capabilities: null, loading: false, error: null, refresh: vi.fn() }),
+}));
+vi.mock("../../hooks/useViewportMode", () => ({ useViewportMode: () => "desktop" }));
+vi.mock("../../hooks/useMobileKeyboard", () => ({
+  useMobileKeyboard: () => ({ keyboardOverlap: 0, viewportHeight: null, viewportOffsetTop: 0, keyboardOpen: false }),
+}));
+vi.mock("../../hooks/useMobileScrollLock", () => ({ useMobileScrollLock: vi.fn() }));
+vi.mock("../../hooks/useConfirm", () => ({ useConfirm: () => ({ confirm: vi.fn() }) }));
+vi.mock("../../hooks/useWorkspaceFileBrowser", () => ({
+  useWorkspaceFileBrowser: () => ({ entries: [], currentPath: ".", setPath: vi.fn(), loading: false, error: null, refresh: vi.fn() }),
+}));
+vi.mock("../../hooks/useWorktrunkInstallStatus", () => ({
+  useWorktrunkInstallStatus: () => ({ status: "idle", requestInstall: vi.fn() }),
+}));
+
+function buildSettings(testMode: boolean) {
+  return {
+    autoMerge: true,
+    testMode,
+    maxConcurrent: 2,
+    maxTriageConcurrent: 2,
+    maxWorktrees: 4,
+    pollIntervalMs: 15000,
+    heartbeatMultiplier: 1,
+    groupOverlappingFiles: true,
+    overlapIgnorePaths: [],
+    mergeStrategy: "direct",
+    mergeIntegrationWorktree: "reuse-task-worktree",
+    recycleWorktrees: false,
+    executorAllowSiblingBranchRename: false,
+    worktreeNaming: "random",
+    worktreesDir: "",
+    worktrunk: { enabled: false, binaryPath: "", onFailure: "fail" },
+    includeTaskIdInCommit: true,
+    ntfyEnabled: false,
+    failureNotificationMode: "sticky-only",
+    failureNotificationDelayMs: 30000,
+    webhookEnabled: false,
+    experimentalFeatures: {},
+  };
+}
+
+describe("SettingsModal testMode toggle", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    const merged = buildSettings(true);
+    mockFetchSettings.mockResolvedValue(merged);
+    mockFetchSettingsByScope.mockResolvedValue({ global: {}, project: { testMode: true } });
+  });
+
+  it("renders merge test mode toggle with initial checked state", async () => {
+    render(<SettingsModal onClose={() => {}} addToast={() => {}} initialSection="merge" />);
+
+    const toggle = await screen.findByLabelText("Enable test mode");
+    expect(toggle).toBeChecked();
+  });
+
+  it("flips checkbox state when clicked", async () => {
+    render(<SettingsModal onClose={() => {}} addToast={() => {}} initialSection="merge" />);
+
+    const toggle = await screen.findByLabelText("Enable test mode");
+    expect(toggle).toBeChecked();
+
+    await userEvent.click(toggle);
+
+    await waitFor(() => {
+      expect(toggle).not.toBeChecked();
+    });
+  });
+});