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/useSettings.test.tsx

@@ -275,4 +275,42 @@ describe("useSettings hook", () => {
     );
     expect(metadataMock.setRequiresRestart).toHaveBeenCalledWith(false);
   });
+
+  it("returns null immediately when settings state is missing", async () => {
+    settingsFormMock = createSettingsFormMock({
+      settings: null,
+    });
+
+    const { result } = renderHook(() => useSettings());
+
+    let resultValue: { requiresRestart: boolean } | null = null;
+    await act(async () => {
+      resultValue = await result.current.saveSettings();
+    });
+
+    expect(resultValue).toBeNull();
+    expect(mutateAsyncMock).not.toHaveBeenCalled();
+    expect(setAppConfigDirOverrideMock).not.toHaveBeenCalled();
+  });
+
+  it("throws when save mutation rejects and keeps restart flag untouched", async () => {
+    settingsFormMock = createSettingsFormMock();
+    directorySettingsMock = createDirectorySettingsMock({
+      appConfigDir: "/override/app",
+      initialAppConfigDir: "/override/app",
+    });
+    const rejection = new Error("save failed");
+    mutateAsyncMock.mockRejectedValueOnce(rejection);
+
+    const { result } = renderHook(() => useSettings());
+
+    await expect(
+      act(async () => {
+        await result.current.saveSettings();
+      }),
+    ).rejects.toThrow("save failed");
+
+    expect(setAppConfigDirOverrideMock).not.toHaveBeenCalled();
+    expect(metadataMock.setRequiresRestart).not.toHaveBeenCalledWith(true);
+  });
 });