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/components/EventCard.tsx

@@ -19,7 +19,7 @@ export const EventCard = ({ event, onEdit, onDelete }: EventCardProps) => {
         startTime={event.occurrenceStart}
         endTime={event.occurrenceEnd}
         description={event.description}
-        isRecurring={event.isRecurring}
+        isRecurring={!!event.recurrenceRule}
       >
         {/* Action buttons - TouchableOpacity with delayPressIn allows ScrollView to detect scroll gestures */}
         <View className="flex-row justify-end mt-3 gap-3">