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/test/setup.ts

@@ -1,6 +1,13 @@
 import "@testing-library/jest-dom/vitest";
 import { vi } from "vitest";
 
+// jsdom does not provide ResizeObserver (used by CalendarPage for sticky block height).
+global.ResizeObserver = vi.fn().mockImplementation(() => ({
+  observe: vi.fn(),
+  unobserve: vi.fn(),
+  disconnect: vi.fn(),
+}));
+
 // jsdom does not provide window.matchMedia (used by use-media-query and use-telegram-theme).
 Object.defineProperty(window, "matchMedia", {
   writable: true,

webapp-next/src/test/test-utils.tsx

@@ -18,6 +18,7 @@ export function resetAppStore() {
   state.setAccessDenied(false);
   state.setCurrentView("calendar");
   state.setSelectedDay(null);
+  state.batchUpdate({ dataForMonthKey: null });
 }
 
 function AllTheProviders({ children }: { children: React.ReactNode }) {