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/example/macos/Podfile.lock

@@ -0,0 +1,22 @@
+PODS:
+  - FlutterMacOS (1.0.0)
+  - path_provider_macos (0.0.1):
+    - FlutterMacOS
+
+DEPENDENCIES:
+  - FlutterMacOS (from `Flutter/ephemeral`)
+  - path_provider_macos (from `Flutter/ephemeral/.symlinks/plugins/path_provider_macos/macos`)
+
+EXTERNAL SOURCES:
+  FlutterMacOS:
+    :path: Flutter/ephemeral
+  path_provider_macos:
+    :path: Flutter/ephemeral/.symlinks/plugins/path_provider_macos/macos
+
+SPEC CHECKSUMS:
+  FlutterMacOS: ae6af50a8ea7d6103d888583d46bd8328a7e9811
+  path_provider_macos: 3c0c3b4b0d4a76d2bf989a913c2de869c5641a19
+
+PODFILE CHECKSUM: 6eac6b3292e5142cfc23bdeb71848a40ec51c14c
+
+COCOAPODS: 1.11.3