fix: eliminate pull-to-refresh flicker by holding stale data during re-fetch

lib/providers/calendar_provider.dart

@@ -56,6 +56,17 @@ class CalendarNotifier extends AsyncNotifier<CalendarState> {
     );
   }
 
+  /// Re-fetch events for the current range without clearing state (no flicker).
+  Future<void> refresh() async {
+    final current = state.value;
+    if (current == null) return;
+    final events = await ref.read(eventsApiProvider).getEvents(
+      current.loadedRange.start,
+      current.loadedRange.end,
+    );
+    state = AsyncData(current.copyWith(eventsByDay: _groupByDay(events)));
+  }
+
   /// Synchronously updates selectedDay and focusedMonth. No API call.
   void selectDay(DateTime day) {
     final current = state.value;