Initial date planner app (#2390)

Initial **experimental** date planner iOS sample.

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

experimental/date_planner/lib/task_row.dart

@@ -0,0 +1,59 @@
+// Copyright 2024 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/cupertino.dart';
+
+import 'event_task.dart';
+
+class TaskRow extends StatefulWidget {
+  final EventTask task;
+  final bool isEditing;
+  const TaskRow({super.key, required this.task, required this.isEditing});
+
+  @override
+  State<TaskRow> createState() => _TaskRowState();
+}
+
+class _TaskRowState extends State<TaskRow> {
+  final _taskText = TextEditingController();
+
+  @override
+  void initState() {
+    _taskText.text = widget.task.text;
+    super.initState();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Row(
+      children: [
+        CupertinoButton(
+          onPressed: widget.isEditing
+              ? () {
+                  setState(() {
+                    widget.task.isCompleted = !widget.task.isCompleted;
+                  });
+                }
+              : null,
+          child: Icon(
+            widget.task.isCompleted
+                ? CupertinoIcons.checkmark_circle_fill
+                : CupertinoIcons.circle,
+            color: CupertinoColors.black,
+          ),
+        ),
+        Expanded(
+          child: widget.isEditing
+              ? CupertinoTextField(
+                  decoration: null,
+                  padding: EdgeInsets.zero,
+                  controller: _taskText,
+                  onChanged: (value) => widget.task.text = value,
+                )
+              : Text(widget.task.text),
+        ),
+      ],
+    );
+  }
+}