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/controller/firebase_options.dart

@@ -0,0 +1,55 @@
+import 'package:firebase_core/firebase_core.dart' show FirebaseOptions;
+import 'package:flutter/foundation.dart' show defaultTargetPlatform, kIsWeb, TargetPlatform;
+
+class DefaultFirebaseOptions {
+  static FirebaseOptions get currentPlatform {
+    if (kIsWeb) {
+      throw UnsupportedError(
+        'DefaultFirebaseOptions have not been configured for web - '
+        'you can reconfigure this by running the FlutterFire CLI again.',
+      );
+    }
+    switch (defaultTargetPlatform) {
+      case TargetPlatform.android:
+        return android;
+      case TargetPlatform.iOS:
+        return ios;
+      case TargetPlatform.macOS:
+        throw UnsupportedError(
+          'DefaultFirebaseOptions have not been configured for macos - '
+          'you can reconfigure this by running the FlutterFire CLI again.',
+        );
+      case TargetPlatform.windows:
+        throw UnsupportedError(
+          'DefaultFirebaseOptions have not been configured for windows - '
+          'you can reconfigure this by running the FlutterFire CLI again.',
+        );
+      case TargetPlatform.linux:
+        throw UnsupportedError(
+          'DefaultFirebaseOptions have not been configured for linux - '
+          'you can reconfigure this by running the FlutterFire CLI again.',
+        );
+      default:
+        throw UnsupportedError(
+          'DefaultFirebaseOptions are not supported for this platform.',
+        );
+    }
+  }
+
+  static const FirebaseOptions android = FirebaseOptions(
+    apiKey: 'AIzaSyCw-ohvxFisvm3Yrb4kCzPBuL0YfZkzI9Q',
+    appId: '1:299075855003:android:292449dc34ad498109dc3e',
+    messagingSenderId: '299075855003',
+    projectId: 'im-fer-c3347',
+    storageBucket: 'im-fer-c3347.firebasestorage.app',
+  );
+
+  static const FirebaseOptions ios = FirebaseOptions(
+    apiKey: 'AIzaSyC0BhV-mo84LvOuqWf2bS966jlf0f8mh74',
+    appId: '1:299075855003:ios:23b06300e33d984309dc3e',
+    messagingSenderId: '299075855003',
+    projectId: 'im-fer-c3347',
+    storageBucket: 'im-fer-c3347.firebasestorage.app',
+    iosBundleId: 'io.openim.flutter.full.NotificationService',
+  );
+}