test: add directory browsing/reset and useSettings hook tests

SettingsDialog Integration Tests:
- Add test for directory browsing and reset functionality
  - Verify app config directory browse/reset flow
  - Verify Claude config directory manual change, browse, and reset
  - Test multiple directory inputs with getAllByTitle pattern
- Add test for export failure error handling
  - User cancels file selection (save_file_dialog returns null)
  - Export operation fails (disk full scenario)
  - Use server.use() to dynamically override MSW handlers
  - Verify toast error messages match i18n keys

MSW Handler Extension:
- Add pick_directory handler to support directory selection API
- Consistent with select_config_directory mock strategy

useSettings Hook Unit Tests:
- Add comprehensive tests for settings save logic
  - Test restart flag when app config directory changes
  - Test no restart when directory unchanged
  - Verify space trimming and empty string to undefined conversion
  - Test Claude plugin sync failure tolerance
- Add test for settings reset functionality
  - Verify form/language/directories reset with server data
- Use factory functions for mock creation (reusability)
- Complete dependency isolation with mock hooks

All tests passing: 9/9 (5 integration + 4 unit tests)

tests/msw/handlers.ts

@@ -138,6 +138,11 @@ export const handlers = [
     return success(default_path ? `${default_path}/picked` : "/mock/selected-dir");
   }),
 
+  http.post(`${TAURI_ENDPOINT}/pick_directory`, async ({ request }) => {
+    const { default_path } = await withJson<{ default_path?: string }>(request);
+    return success(default_path ? `${default_path}/picked` : "/mock/selected-dir");
+  }),
+
   http.post(`${TAURI_ENDPOINT}/open_file_dialog`, () =>
     success("/mock/import-settings.json"),
   ),