feat: update loading state handling in DutyList component

- Replaced the loading skeleton with a compact loading placeholder to improve user experience when data is not yet loaded for the month.
- Enhanced the rendering logic to ensure the loading state is visually distinct and does not display the skeleton when data is being fetched.
- Updated related tests to verify the new loading behavior and ensure accurate feedback during data fetching.

webapp-next/src/components/duty/DutyList.test.tsx

@@ -42,12 +42,13 @@ describe("DutyList", () => {
     expect(screen.getByText(/No duties this month/i)).toBeInTheDocument();
   });
 
-  it("renders quiet placeholder when data not yet loaded for month", () => {
+  it("renders compact loading placeholder when data not yet loaded for month (no skeleton)", () => {
     useAppStore.getState().setDuties([]);
     useAppStore.getState().batchUpdate({ dataForMonthKey: null });
     render(<DutyList />);
     expect(screen.queryByText(/No duties this month/i)).not.toBeInTheDocument();
     expect(document.querySelector('[aria-busy="true"]')).toBeInTheDocument();
+    expect(document.querySelector('[data-slot="skeleton"]')).not.toBeInTheDocument();
   });
 
   it("renders duty with full_name and time range", () => {