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/lib/widgets/register_page_bg.dart

@@ -0,0 +1,39 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_screenutil/flutter_screenutil.dart';
+import 'package:get/get.dart';
+import 'package:openim_common/openim_common.dart';
+
+class RegisterBgView extends StatelessWidget {
+  const RegisterBgView({
+    Key? key,
+    required this.child,
+  }) : super(key: key);
+  final Widget child;
+
+  @override
+  Widget build(BuildContext context) => Material(
+        child: TouchCloseSoftKeyboard(
+          isGradientBg: true,
+          child: SingleChildScrollView(
+            child: Column(
+              crossAxisAlignment: CrossAxisAlignment.start,
+              children: [
+                54.verticalSpace,
+                Padding(
+                  padding: EdgeInsets.only(left: 22.w),
+                  child: ImageRes.backBlack.toImage
+                    ..width = 24.w
+                    ..height = 24.h
+                    ..onTap = () => Get.back(),
+                ),
+                38.verticalSpace,
+                Padding(
+                  padding: EdgeInsets.symmetric(horizontal: 32.w),
+                  child: child,
+                ),
+              ],
+            ),
+          ),
+        ),
+      );
+}