feat(client): 桌面端日志加本地时间戳前缀 [HH:MM:SS.mmm]

新增 log_time.dart(共享 logTime() helper),DesktopKernel / DesktopVpnBridge
所有日志行首打 [时:分:秒.毫秒],便于排查连接时序。

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

client/lib/bridge/kernel_process.dart

@@ -30,6 +30,7 @@ import 'dart:math' as math;
 
 import 'package:http/http.dart' as http;
 
+import 'log_time.dart';
 import 'vpn_bridge.dart';
 
 // ══════════════════════════════════════════════════════════════════
@@ -481,7 +482,7 @@ class DesktopKernelProcess implements KernelProcess {
   }
 
   void _log(String msg) {
-    print('[DesktopKernel] $msg');
+    print('${logTime()}[DesktopKernel] $msg');
     if (!_logCtrl.isClosed) _logCtrl.add(msg);
   }