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/CHANGELOG.md

@@ -0,0 +1,24 @@
+## [0.5.5] - Bug Fixes
+
+* Update dio to 5.0.1 #10 mbfakourii 
+* Replace slash with Platform.pathSeparator on downloader #12 (Paulo Ortolan)
+* Fixed download task list #4 (arjundevlucid)
+* Fixed initial progress error during resume, after multiple pause resumes (YanRui)
+
+## [0.5.4] - Bug Fixes
+
+* Improve batch progress notification to notify more granular
+
+## [0.5.3] - Bug Fixes
+
+* Disable Streams temporarily to fix bad state bugs
+* Bug Fix: Stop Same Download Requests if already downloading
+* Return DownloadTask  when addDownload
+
+## [0.5.2] - Added Stream to expose events for all task when download status changes
+
+* Added Stream to expose events for all task when download status changes
+
+## [0.5.0] - Initial Release
+
+* Introducing Flutter Download Manager.