feat: implement pending month handling in calendar components

- Introduced a new `pendingMonth` state in the app store to manage month transitions without clearing current data, enhancing user experience during month navigation.
- Updated `useMonthData` hook to load data for the `pendingMonth` when set, preventing empty-frame flicker and ensuring smooth month switching.
- Modified `CalendarPage` and `CalendarGrid` components to utilize the new `pendingMonth` state, improving the rendering logic during month changes.
- Enhanced `DutyList` to display a loading skeleton while data is being fetched, providing better feedback to users.
- Updated relevant tests to cover the new loading behavior and state management for month transitions.

webapp-next/src/components/CalendarPage.tsx

@@ -48,6 +48,7 @@ export function CalendarPage({ isAllowed, initDataRaw }: CalendarPageProps) {
 
   const {
     currentMonth,
+    pendingMonth,
     loading,
     error,
     accessDenied,
@@ -62,6 +63,7 @@ export function CalendarPage({ isAllowed, initDataRaw }: CalendarPageProps) {
   } = useAppStore(
     useShallow((s) => ({
       currentMonth: s.currentMonth,
+      pendingMonth: s.pendingMonth,
       loading: s.loading,
       error: s.error,
       accessDenied: s.accessDenied,