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/ios/NotificationService/Info.plist

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>NSExtension</key>
+	<dict>
+		<key>NSExtensionPointIdentifier</key>
+		<string>com.apple.usernotifications.service</string>
+		<key>NSExtensionPrincipalClass</key>
+		<string>$(PRODUCT_MODULE_NAME).NotificationService</string>
+	</dict>
+</dict>
+</plist>

mobile-next/ios/NotificationService/NotificationService.entitlements

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>com.apple.security.application-groups</key>
+	<array>
+		<string>group.cn.rentsoft.openim.flutter.rtc</string>
+	</array>
+</dict>
+</plist>

mobile-next/ios/NotificationService/NotificationService.swift

@@ -0,0 +1,33 @@
+import UserNotifications
+
+class NotificationService: UNNotificationServiceExtension {
+    var contentHandler: ((UNNotificationContent) -> Void)?
+    var bestAttemptContent: UNMutableNotificationContent?
+    
+    override func didReceive(_ request: UNNotificationRequest, withContentHandler contentHandler: @escaping (UNNotificationContent) -> Void) {
+        self.contentHandler = contentHandler
+        bestAttemptContent = (request.content.mutableCopy() as? UNMutableNotificationContent)
+        
+        if let bestAttemptContent = bestAttemptContent {
+            // Modify the notification content here...
+            if let userInfo = request.content.userInfo as? [String: Any],
+               let clientID = userInfo["clientMsgID"] as? String,
+               !clientID.isEmpty == true
+            {
+                let key = "key"
+                UserDefaults.standard.set(request.identifier, forKey: key)
+                bestAttemptContent.title = "\(bestAttemptContent.title)"
+                bestAttemptContent.sound = UNNotificationSound(named: .init(rawValue: "call.caf"))
+            }
+            contentHandler(bestAttemptContent)
+        }
+    }
+    
+    override func serviceExtensionTimeWillExpire() {
+        // Called just before the extension will be terminated by the system.
+        // Use this as an opportunity to deliver your "best attempt" at modified content, otherwise the original push payload will be used.
+        if let contentHandler = contentHandler, let bestAttemptContent = bestAttemptContent {
+            contentHandler(bestAttemptContent)
+        }
+    }
+}