feat(mobile-next): 导入官方 openim-flutter-demo 固定基线 3.8.3-patch.3 并完成双构建

- 上游: github.com/OpenIMSDK/openim-flutter-demo tag 3.8.3-patch.3
  commit b3dfdb1e8aaeaaf6f0793e10cadd20d5c184a31f
- 并行目录 mobile-next/ 原样导入（含 LICENSE），不覆盖现网 mobile/
- 锁定 Flutter 3.24.5 / Dart 3.5.4 / JDK 17 / Gradle 7.6.3 / AGP 7.3.1
- 实测 Android debug 与 release 构建均成功（见 docs/mobile-next-baseline-import.md）
- 本提交可单独回退：git revert 1db1229（重写前）

Co-authored-by: multica-agent <github@multica.ai>

mobile-next/lib/pages/contacts/add_method/add_method_logic.dart

@@ -0,0 +1,17 @@
+import 'package:flutter_openim_sdk/flutter_openim_sdk.dart';
+import 'package:get/get.dart';
+import 'package:openim/pages/contacts/add_by_search/add_by_search_logic.dart';
+import 'package:openim/routes/app_navigator.dart';
+
+class AddContactsMethodLogic extends GetxController {
+  scan() => AppNavigator.startScan();
+
+  addFriend() =>
+      AppNavigator.startAddContactsBySearch(searchType: SearchType.user);
+
+  createGroup() => AppNavigator.startCreateGroup(
+      defaultCheckedList: [OpenIM.iMManager.userInfo]);
+
+  addGroup() =>
+      AppNavigator.startAddContactsBySearch(searchType: SearchType.group);
+}