refactor(DiaryDateActivityService): improve orderId calculation logic for group activities

- Updated the logic for determining the maximum orderId to handle cases with and without groupId more clearly.
- Enhanced code readability by consolidating the where clause for fetching max orderId based on group context.

backend/services/diaryDateActivityService.js

@@ -64,11 +64,17 @@ class DiaryDateActivityService {
             }
         }
         
-        const maxOrderId = await DiaryDateActivity.max('orderId', {
-            where: {
+        const maxOrderWhere = restData.groupId
+            ? {
                 diaryDateId: data.diaryDateId,
                 groupId: restData.groupId
             }
+            : {
+                // Globale Aktivitäten bei "Alle" sollen am Ende des gesamten Tagesplans landen.
+                diaryDateId: data.diaryDateId
+            };
+        const maxOrderId = await DiaryDateActivity.max('orderId', {
+            where: maxOrderWhere
         });
         const newOrderId = maxOrderId !== null ? maxOrderId + 1 : 1;
         restData.orderId = newOrderId;