feat(holiday-system): add holiday population functionality and API integration

- Implemented a new API endpoint to populate country holidays in the database.
- Enhanced the HolidayController to handle holiday population logic for multiple countries.
- Updated the holiday API router to include the new populate endpoint.
- Added a service method in the frontend to trigger holiday population.
- Integrated a button in the admin center overview for easy access to populate holidays.
- Improved error handling and user feedback during the holiday population process.

worklenz-frontend/src/api/holiday/holiday.api.service.ts

@@ -65,4 +65,10 @@ export const holidayApiService = {
     const response = await apiClient.get<IServerResponse<IHolidayCalendarEvent[]>>(`${rootUrl}/calendar?year=${year}&month=${month}`);
     return response.data;
   },
+
+  // Populate holidays
+  populateCountryHolidays: async (): Promise<IServerResponse<any>> => {
+    const response = await apiClient.post<IServerResponse<any>>(`${rootUrl}/populate`);
+    return response.data;
+  },
 };