refactor: remove redundant isRecurring property, use recurrenceRule instead

isRecurring was redundant since recurrenceRule as truthy/falsy check suffices.
Removed from shared CalendarEvent type, Mongoose virtual, and all usages.

apps/client/src/app/(tabs)/calendar.tsx

@@ -203,7 +203,7 @@ const Calendar = () => {
     const occurrenceDate = getDateKey(new Date(event.occurrenceStart));
 
     try {
-      if (event.isRecurring) {
+      if (event.recurrenceRule) {
         // Recurring event: use mode and occurrenceDate
         await EventService.delete(event.id, mode, occurrenceDate);
         // Reload events to reflect changes
@@ -260,7 +260,7 @@ const Calendar = () => {
       <DeleteEventModal
         visible={deleteModalVisible}
         eventTitle={eventToDelete?.title || ""}
-        isRecurring={eventToDelete?.isRecurring || false}
+        isRecurring={!!eventToDelete?.recurrenceRule}
         onConfirm={handleDeleteConfirm}
         onCancel={handleDeleteCancel}
       />