feat: Notifications page, Sidebar layout fixes, and Achievements enhancements

- implemented /notifications page with overdue/upcoming alerts
- Fixed sidebar scrolling in collapsed mode
- Moved notification button to sidebar footer
- Enhanced Achievements page with streak stats and tooltips
- Improved XP history to show task titles
- Added missing translations (en/de)
- Removed top bar header
- Fixed Docker environment routing

client/src/components/TaskDetailsModal.tsx

@@ -66,9 +66,10 @@ export default function TaskDetailsModal({
   const [isCreatingSubtask, setIsCreatingSubtask] = useState(false);
   const [isScheduling, setIsScheduling] = useState(false);
 
-  const { data: tasks = [] } = useQuery<Task[]>({
+  const { data: tasksData } = useQuery<Task[]>({
     queryKey: ['/api/tasks'],
   });
+  const tasks = tasksData ?? [];
 
   // Notes state
   const [notes, setNotes] = useState('');