test: add error handling and edge case tests for hooks

useSettings Tests:
- Add test for null settings state protection
  - Returns null immediately without calling APIs
  - Prevents null pointer errors in save flow
- Add test for save mutation failure
  - Verifies error propagates to caller
  - Ensures restart flag remains untouched on failure
  - Validates no side effects when save fails

useProviderActions Tests:
- Add error propagation tests for CRUD operations
  - updateProvider: propagates errors to caller
  - addProvider: propagates errors to caller
  - deleteProvider: propagates errors to caller
- Add switch mutation error handling tests
  - Claude switch: handles errors silently (no throw)
  - Codex switch: skips plugin sync when mutation fails
  - Verifies plugin sync APIs not called on failure
- Add loading state verification
  - Test all mutations pending scenario (existing)
  - Test all mutations idle scenario (new)
  - Ensures isLoading flag accuracy

useImportExport Edge Case Tests (new file):
- Add test for user cancelling file dialog
  - File dialog returns null (user cancelled)
  - State remains unchanged (selectedFile: "", status: "idle")
  - No error toast shown
- Add test for resetStatus behavior
  - Clears error message and status
  - Preserves selected file path for retry
  - Resets backupId to null

All tests passing: 79/79 (10 new tests)

tests/hooks/useImportExport.extra.test.tsx

@@ -0,0 +1,76 @@
+import { renderHook, act } from "@testing-library/react";
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { useImportExport } from "@/hooks/useImportExport";
+
+const toastSuccessMock = vi.fn();
+const toastErrorMock = vi.fn();
+
+vi.mock("sonner", () => ({
+  toast: {
+    success: (...args: unknown[]) => toastSuccessMock(...args),
+    error: (...args: unknown[]) => toastErrorMock(...args),
+  },
+}));
+
+const openFileDialogMock = vi.fn();
+const importConfigMock = vi.fn();
+
+vi.mock("@/lib/api", () => ({
+  settingsApi: {
+    openFileDialog: (...args: unknown[]) => openFileDialogMock(...args),
+    importConfigFromFile: (...args: unknown[]) => importConfigMock(...args),
+    saveFileDialog: vi.fn(),
+    exportConfigToFile: vi.fn(),
+  },
+}));
+
+describe("useImportExport Hook (edge cases)", () => {
+  beforeEach(() => {
+    openFileDialogMock.mockReset();
+    importConfigMock.mockReset();
+    toastSuccessMock.mockReset();
+    toastErrorMock.mockReset();
+    vi.useFakeTimers();
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it("keeps state unchanged when file dialog resolves to null", async () => {
+    openFileDialogMock.mockResolvedValue(null);
+    const { result } = renderHook(() => useImportExport());
+
+    await act(async () => {
+      await result.current.selectImportFile();
+    });
+
+    expect(result.current.selectedFile).toBe("");
+    expect(result.current.status).toBe("idle");
+    expect(toastErrorMock).not.toHaveBeenCalled();
+  });
+
+  it("resetStatus clears errors but preserves selected file", async () => {
+    openFileDialogMock.mockResolvedValue("/config.json");
+    importConfigMock.mockResolvedValue({ success: false, message: "broken" });
+    const { result } = renderHook(() => useImportExport());
+
+    await act(async () => {
+      await result.current.selectImportFile();
+    });
+
+    await act(async () => {
+      await result.current.importConfig();
+    });
+
+    act(() => {
+      result.current.resetStatus();
+    });
+
+    expect(result.current.selectedFile).toBe("/config.json");
+    expect(result.current.status).toBe("idle");
+    expect(result.current.errorMessage).toBeNull();
+    expect(result.current.backupId).toBeNull();
+  });
+
+});