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/pages/splash/splash_view.dart

@@ -0,0 +1,36 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_screenutil/flutter_screenutil.dart';
+import 'package:get/get.dart';
+import 'package:openim_common/openim_common.dart';
+
+import 'splash_logic.dart';
+
+class SplashPage extends StatelessWidget {
+  final logic = Get.find<SplashLogic>();
+
+  SplashPage({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      decoration: BoxDecoration(
+        gradient: LinearGradient(
+          colors: [Styles.c_0089FF_opacity10, Styles.c_FFFFFF_opacity0],
+          begin: Alignment.topCenter,
+          end: Alignment.bottomCenter,
+        ),
+      ),
+      child: Stack(
+        alignment: Alignment.center,
+        children: [
+          Positioned(
+            bottom: 130.h,
+            child: ImageRes.splashLogo.toImage
+              ..width = 55.61.w
+              ..height = 78.91.h,
+          ),
+        ],
+      ),
+    );
+  }
+}