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/local_plugin/flutter_download_manager/pubspec.yaml

@@ -0,0 +1,22 @@
+name: flutter_download_manager
+description: Flutter Download Manager is a Cross-Platform file downloader with resumeable, parallel and batch Download support. Also be notified of download status and progress updates
+version: 0.5.5
+homepage: https://github.com/nabil6391/flutter_download_manager
+
+environment:
+  sdk: '>=2.12.0 <3.0.0'
+  flutter: ">=1.17.0"
+
+dependencies:
+  flutter:
+    sdk: flutter
+  dio:
+  collection:
+
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+  lints: ^3.0.0
+
+flutter:
+