added checkbox to toggle if task is completed

lib/pages/task_overview_page.dart

@@ -40,7 +40,13 @@ class _TaskOverviewPageState extends State<TaskOverviewPage> {
     return ListTile(
       key: Key(task.id),
       title: Text(task.title),
-      subtitle: Text(task.description),
+      subtitle: task.description.isNotEmpty ? Text(task.description) : null,
+      trailing: Checkbox(
+        value: task.isCompleted,
+        onChanged: (isCompleted) => context
+            .controller<TaskController>()
+            .saveTask(task.copyWith(isCompleted: isCompleted)),
+      ),
       onTap: () async {
         final result = await onTaskTapped(task);
         if (result != null && context.mounted) {