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/main.dart

@@ -0,0 +1,19 @@
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+import 'package:openim_common/openim_common.dart';
+
+import 'app.dart';
+
+void main() {
+  runZonedGuarded(() {
+    FlutterError.onError = (FlutterErrorDetails details) {
+      FlutterError.presentError(details);
+      Logger.print('FlutterError: ${details.exception.toString()}, ${details.stack.toString()}');
+    };
+
+    Config.init(() => runApp(const ChatApp()));
+  }, (error, stackTrace) {
+    Logger.print('FlutterError: ${error.toString()}, ${stackTrace.toString()}');
+  });
+}