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/lib/src/widgets/chat/chat_bubble.dart

@@ -0,0 +1,42 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_screenutil/flutter_screenutil.dart';
+import 'package:openim_common/openim_common.dart';
+
+enum BubbleType {
+  send,
+  receiver,
+}
+
+class ChatBubble extends StatelessWidget {
+  const ChatBubble({
+    Key? key,
+    this.margin,
+    this.constraints,
+    this.alignment = Alignment.center,
+    this.backgroundColor,
+    this.child,
+    required this.bubbleType,
+  }) : super(key: key);
+  final EdgeInsetsGeometry? margin;
+  final BoxConstraints? constraints;
+  final AlignmentGeometry? alignment;
+  final Color? backgroundColor;
+  final Widget? child;
+  final BubbleType bubbleType;
+
+  bool get isISend => bubbleType == BubbleType.send;
+
+  @override
+  Widget build(BuildContext context) => Container(
+        constraints: constraints,
+        margin: margin,
+        padding: EdgeInsets.symmetric(horizontal: 12.w, vertical: 10.h),
+        alignment: alignment,
+        decoration: BoxDecoration(
+          color:
+              backgroundColor ?? (isISend ? Styles.c_CCE7FE : Styles.c_F4F5F7),
+          borderRadius: borderRadius(isISend),
+        ),
+        child: child,
+      );
+}