test(TaskListScreen): add integration tests for filtered and overdue task states

- Covers empty states, celebration state, and scheduled/overdue task rendering
- Verifies proper checkbox behavior for future tasks
- Tests AppBar for sort dropdown, edit/delete actions, and calendar strip
- Adds necessary test helpers and overrides for room-specific tasks

lib/features/tasks/presentation/task_row.dart

@@ -42,6 +42,7 @@ class TaskRow extends ConsumerWidget {
       task.nextDueDate.day,
     );
     final isOverdue = dueDate.isBefore(today);
+    final isFuture = dueDate.isAfter(today);
 
     // Format relative due date in German
     final relativeDateText = formatRelativeDate(task.nextDueDate, now);
@@ -56,10 +57,12 @@ class TaskRow extends ConsumerWidget {
     return ListTile(
       leading: Checkbox(
         value: false, // Always unchecked -- completion is immediate + reschedule
-        onChanged: (_) {
-          // Mark done immediately (optimistic UI, no undo per user decision)
-          ref.read(taskActionsProvider.notifier).completeTask(task.id);
-        },
+        onChanged: isFuture
+            ? null // Future tasks cannot be completed yet
+            : (_) {
+                // Mark done immediately (optimistic UI, no undo per user decision)
+                ref.read(taskActionsProvider.notifier).completeTask(task.id);
+              },
       ),
       title: Text(
         task.name,