Cupertino gallery app (#2715)

Resolves #2519 

Part of the larger effort to bring the samples/repo into a stable and
maintainable condition that _also_ remains useful. See: #2409

## 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 have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

cupertino_gallery/lib/main.dart

@@ -0,0 +1,72 @@
+// Copyright 2022 The Flutter team. 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 'package:flutter/material.dart';
+
+import 'gallery_home.dart';
+
+void main() {
+  runApp(const CupertinoGalleryApp());
+}
+
+class CupertinoGalleryApp extends StatefulWidget {
+  const CupertinoGalleryApp({super.key});
+
+  @override
+  State<CupertinoGalleryApp> createState() => _CupertinoGalleryAppState();
+}
+
+class _CupertinoGalleryAppState extends State<CupertinoGalleryApp> {
+  ThemeMode _themeMode = ThemeMode.system;
+  double _textSize = 1.0;
+
+  void _handleThemeChange(bool isDarkMode) {
+    setState(() {
+      _themeMode = isDarkMode ? ThemeMode.dark : ThemeMode.light;
+    });
+  }
+
+  void _handleTextSizeChange(double newTextSize) {
+    setState(() {
+      _textSize = newTextSize;
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final baseTheme = CupertinoThemeData(
+      brightness:
+          _themeMode == ThemeMode.dark ? Brightness.dark : Brightness.light,
+    );
+    final textTheme = baseTheme.textTheme.copyWith(
+      textStyle:
+          baseTheme.textTheme.textStyle.copyWith(fontSize: 14 * _textSize),
+      actionTextStyle: baseTheme.textTheme.actionTextStyle
+          .copyWith(fontSize: 14 * _textSize),
+      tabLabelTextStyle: baseTheme.textTheme.tabLabelTextStyle
+          .copyWith(fontSize: 10 * _textSize),
+      navTitleTextStyle: baseTheme.textTheme.navTitleTextStyle
+          .copyWith(fontSize: 17 * _textSize),
+      navLargeTitleTextStyle: baseTheme.textTheme.navLargeTitleTextStyle
+          .copyWith(fontSize: 34 * _textSize),
+      navActionTextStyle: baseTheme.textTheme.navActionTextStyle
+          .copyWith(fontSize: 17 * _textSize),
+      pickerTextStyle: baseTheme.textTheme.pickerTextStyle
+          .copyWith(fontSize: 21 * _textSize),
+      dateTimePickerTextStyle: baseTheme.textTheme.dateTimePickerTextStyle
+          .copyWith(fontSize: 21 * _textSize),
+    );
+    return CupertinoApp(
+      title: 'Cupertino Gallery',
+      theme: baseTheme.copyWith(textTheme: textTheme),
+      home: GalleryHome(
+        onThemeChange: _handleThemeChange,
+        isDarkMode: _themeMode == ThemeMode.dark,
+        onTextSizeChange: _handleTextSizeChange,
+        textSize: _textSize,
+      ),
+    );
+  }
+}