chore: license the project under AGPL-3.0

- 添加官方 AGPL-3.0 全文
- LICENSE 作为 asset 打包，注册进 Flutter 许可清单
- 关于页「开源许可」改为打开许可页
- README 补充简介、开发命令与许可说明

lib/main.dart

@@ -1,13 +1,26 @@
 import 'package:flutter/cupertino.dart' show CupertinoPageTransitionsBuilder;
+import 'package:flutter/foundation.dart'
+    show LicenseEntryWithLineBreaks, LicenseRegistry;
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart' show rootBundle;
 
 import 'app_info.dart';
 import 'app_shell.dart';
 
 void main() {
+  _registerAppLicense();
   runApp(const KiriMemoApp());
 }
 
+/// 把 LICENSE（AGPL-3.0）注册进 Flutter 的许可清单，
+/// 这样「关于 → 开源许可」里会和应用依赖的许可显示在一起。
+void _registerAppLicense() {
+  LicenseRegistry.addLicense(() async* {
+    final String text = await rootBundle.loadString('LICENSE');
+    yield LicenseEntryWithLineBreaks(<String>[appName], text);
+  });
+}
+
 class KiriMemoApp extends StatelessWidget {
   const KiriMemoApp({super.key});
 

lib/pages/about_page.dart

@@ -63,16 +63,15 @@ class AboutPage extends StatelessWidget {
               color: colors.onSurfaceVariant,
             ),
             title: const Text('开源许可'),
-            subtitle: const Text('MIT License'),
+            subtitle: const Text('GNU AGPL v3.0'),
             trailing: Icon(
               Icons.chevron_right_rounded,
               color: colors.onSurfaceVariant,
             ),
-            onTap: () => ScaffoldMessenger.of(context).showSnackBar(
-              const SnackBar(
-                content: Text('许可文本尚未接入'),
-                behavior: SnackBarBehavior.floating,
-              ),
+            onTap: () => showLicensePage(
+              context: context,
+              applicationName: appName,
+              applicationVersion: appVersion,
             ),
           ),
         ],