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/event_editor.dart

@@ -0,0 +1,117 @@
+// 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.dart';
+import 'event_detail.dart';
+
+class EventEditor extends StatefulWidget {
+  final Event event;
+  final bool isNew;
+  const EventEditor({
+    super.key,
+    required this.event,
+    required this.isNew,
+  });
+
+  @override
+  State<EventEditor> createState() => _EventEditorState();
+}
+
+class _EventEditorState extends State<EventEditor> {
+  late Event event;
+  late bool isNew;
+  late bool isEditing;
+
+  @override
+  void initState() {
+    isNew = widget.isNew;
+    isEditing = isNew;
+    event = widget.event;
+    super.initState();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoPageScaffold(
+      backgroundColor: CupertinoColors.secondarySystemBackground,
+      navigationBar: CupertinoNavigationBar(
+        // TODO(mit-mit): Resolve manual padding issues.
+        //
+        // Note that even with the padding overriding below, the chevron/
+        // back arrow doesn't seem to be far enough to the left.
+        //
+        // Is this maybe the issue here?
+        // https://github.com/flutter/flutter/issues/91715
+        leading: isNew
+            ? CupertinoButton(
+                padding: EdgeInsets.zero,
+                child: const Text('Cancel'),
+                onPressed: () => Navigator.pop(context, null),
+              )
+            : CupertinoButton(
+                padding: EdgeInsets.zero,
+                onPressed: () {
+                  Navigator.pop(context, event);
+                },
+                child: const Row(
+                  children: [
+                    Icon(CupertinoIcons.back),
+                    Text('Date Planner'),
+                  ],
+                ),
+              ),
+        trailing: CupertinoButton(
+          padding: EdgeInsets.zero,
+          child: Text(isNew ? 'Add' : (isEditing ? 'Done' : 'Edit')),
+          onPressed: () {
+            if (isNew) {
+              Navigator.pop(context, event);
+            } else {
+              setState(() {
+                if (isEditing) {
+                  isEditing = false;
+                } else {
+                  isEditing = true;
+                }
+              });
+            }
+          },
+        ),
+      ),
+      // TODO(mit-mit): Why isn't SafeArea included by default?
+      child: SafeArea(
+        bottom: false,
+        child: Column(
+          crossAxisAlignment: CrossAxisAlignment.stretch,
+          children: [
+            EventDetail(
+              event: event,
+              isEditing: isEditing,
+            ),
+            const Spacer(),
+            if (isEditing && !isNew)
+              ColoredBox(
+                color: CupertinoColors.white,
+                child: Column(
+                  children: [
+                    CupertinoButton(
+                      child: const Text('Delete Event'),
+                      onPressed: () {
+                        setState(() {
+                          Navigator.pop(context, null);
+                        });
+                      },
+                    ),
+                    const SizedBox(height: 24),
+                  ],
+                ),
+              ),
+          ],
+        ),
+      ),
+    );
+  }
+}