fix: update loading state handling in CalendarPage and DutyList components

- Set isLoading to false in CalendarPage to prevent unnecessary loading indicators.
- Removed the loading spinner from CalendarHeader and adjusted rendering logic in DutyList to show a placeholder when data is not yet loaded for the month.
- Enhanced tests in DutyList to verify behavior when data is empty and when data is loading, ensuring accurate user feedback during data fetching.

webapp-next/src/components/CalendarPage.tsx

@@ -145,7 +145,7 @@ export function CalendarPage({ isAllowed, initDataRaw }: CalendarPageProps) {
       >
         <CalendarHeader
           month={currentMonth}
-          isLoading={loading}
+          isLoading={false}
           disabled={navDisabled}
           onGoToToday={handleGoToToday}
           onRefresh={retry}