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/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,
             ),
           ),
         ],