feat(android): Dart provider 接 Android→VpnNativeBridge(Task 4)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/bridge/vpn_bridge_provider.dart

@@ -1,6 +1,7 @@
 // vpn_bridge_provider.dart — VpnBridge 平台分派
 //
 // macOS / Linux / Windows → DesktopVpnBridge（真实子进程）
+// Android → VpnNativeBridge（内嵌 libbox 的 VpnService，走 pangolin/vpn 通道）
 // 其他平台（iOS / Web / 测试）→ VpnBridgeMock
 import 'package:flutter/foundation.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
@@ -24,6 +25,9 @@ final vpnBridgeProvider = Provider<VpnBridge>((ref) {
     if (Platform.isMacOS && kUseNativeVpnMacOS) {
       return VpnNativeBridge();
     }
+    if (Platform.isAndroid) {
+      return VpnNativeBridge();
+    }
     if (Platform.isMacOS || Platform.isLinux || Platform.isWindows) {
       return DesktopVpnBridge();
     }