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/openim_common/.metadata

@@ -0,0 +1,10 @@
+# This file tracks properties of this Flutter project.
+# Used by Flutter tool to assess capabilities and perform upgrades etc.
+#
+# This file should be version controlled and should not be manually edited.
+
+version:
+  revision: 2ad6cd72c040113b47ee9055e722606a490ef0da
+  channel: stable
+
+project_type: package