feat: enhance CalendarPage and DutyList components for improved loading state handling

- Removed the loading state placeholder from CalendarPage, directly rendering the CalendarGrid component.
- Updated DutyList to display a loading message when data is being fetched, enhancing user experience during data loading.
- Introduced a new dataForMonthKey in the app store to manage month-specific data more effectively.
- Refactored useMonthData hook to reset duties and calendarEvents when a new month is detected, ensuring accurate data representation.
- Added tests to verify the new loading state behavior in both components.

webapp-next/src/store/app-store.ts

@@ -9,11 +9,16 @@ import { getStartParamFromUrl } from "@/lib/launch-params";
 
 export type CurrentView = "calendar" | "currentDuty";
 
+/** YYYY-MM key for the month that duties/calendarEvents belong to; null when none loaded. */
+export type DataForMonthKey = string | null;
+
 export interface AppState {
   currentMonth: Date;
   lang: "ru" | "en";
   duties: DutyWithUser[];
   calendarEvents: CalendarEvent[];
+  /** YYYY-MM: duties and calendarEvents are for this month; null when loading or no data. */
+  dataForMonthKey: DataForMonthKey;
   loading: boolean;
   error: string | null;
   accessDenied: boolean;
@@ -35,7 +40,7 @@ export interface AppState {
   setCurrentView: (v: CurrentView) => void;
   setSelectedDay: (key: string | null) => void;
   /** Batch multiple state updates into a single re-render. */
-  batchUpdate: (partial: Partial<Pick<AppState, "currentMonth" | "lang" | "duties" | "calendarEvents" | "loading" | "error" | "accessDenied" | "accessDeniedDetail" | "currentView" | "selectedDay">>) => void;
+  batchUpdate: (partial: Partial<Pick<AppState, "currentMonth" | "lang" | "duties" | "calendarEvents" | "dataForMonthKey" | "loading" | "error" | "accessDenied" | "accessDeniedDetail" | "currentView" | "selectedDay">>) => void;
 }
 
 const now = new Date();
@@ -52,6 +57,7 @@ export const useAppStore = create<AppState>((set) => ({
   lang: "en",
   duties: [],
   calendarEvents: [],
+  dataForMonthKey: null,
   loading: false,
   error: null,
   accessDenied: false,