refactor(client): 抽统一日志函数 logLine + 去掉重复时间戳

- log_time.dart → log.dart:提供 logLine(tag, msg),格式 `[时:分:秒.毫秒] [tag] msg`,
  DesktopKernel / DesktopVpnBridge 散落的 print+拼接全部收敛到此。
- 下发配置 log.timestamp=false:sing-box 不再自带时间戳,避免一行打印两个时间
  (之前 `[22:10:35.213] [stderr] +0800 2026-06-18 22:10:35 ...` 重复)。

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

client/lib/bridge/desktop_vpn_bridge.dart

@@ -25,7 +25,7 @@ import 'dart:convert';
 import 'dart:io';
 
 import 'kernel_process.dart';
-import 'log_time.dart';
+import 'log.dart';
 import 'vpn_bridge.dart';
 
 /// 桌面端 VPN 桥接实现（macOS / Linux / Windows PoC）。
@@ -101,9 +101,9 @@ class DesktopVpnBridge implements VpnBridge {
 
     // 3. 写 config 到应用支持目录（0600 权限）
     final configPath = await writeConfig(enrichedJson);
-    print('${logTime()}[DesktopVpnBridge] config written: $configPath');
-    print('${logTime()}[DesktopVpnBridge] clash_api port=$port secret_len=${secret.length}');
-    print('${logTime()}[DesktopVpnBridge] killSwitch=$_killSwitchEnabled');
+    logLine('DesktopVpnBridge', 'config written: $configPath');
+    logLine('DesktopVpnBridge', 'clash_api port=$port secret_len=${secret.length}');
+    logLine('DesktopVpnBridge', 'killSwitch=$_killSwitchEnabled');
 
     // 4. 启动内核子进程（blocking until Clash API ready or error）
     await _kernel.spawn(configPath);
@@ -142,7 +142,7 @@ class DesktopVpnBridge implements VpnBridge {
     try {
       await _kernel.clashApiClient.selectProxy('proxy', tag);
     } catch (e) {
-      print('${logTime()}[DesktopVpnBridge] selectOutbound(tag=$tag) error: $e');
+      logLine('DesktopVpnBridge', 'selectOutbound(tag=$tag) error: $e');
       rethrow;
     }
   }
@@ -178,11 +178,11 @@ class DesktopVpnBridge implements VpnBridge {
   Future<void> setKillSwitch({required bool on}) async {
     if (_killSwitchEnabled == on) return;
     _killSwitchEnabled = on;
-    print('${logTime()}[DesktopVpnBridge] setKillSwitch=$on');
+    logLine('DesktopVpnBridge', 'setKillSwitch=$on');
 
     // 若内核正在运行，用新设置重载（会有约 1~2s 重连）
     if (_kernel.isRunning && _lastUserConfigJson != null) {
-      print('${logTime()}[DesktopVpnBridge] reloading kernel for killSwitch change');
+      logLine('DesktopVpnBridge', 'reloading kernel for killSwitch change');
       // 暂停自动重连，避免 kill 触发重连定时器
       final wasAutoReconnect = _shouldAutoReconnect;
       _shouldAutoReconnect = false;
@@ -230,18 +230,18 @@ class DesktopVpnBridge implements VpnBridge {
         _reconnectAttempt.clamp(0, _kRetryDelaysSec.length - 1)];
     _reconnectAttempt++;
 
-    print(
-        '${logTime()}[DesktopVpnBridge] auto-reconnect in ${delaySec}s (attempt $_reconnectAttempt)');
+    logLine('DesktopVpnBridge',
+        'auto-reconnect in ${delaySec}s (attempt $_reconnectAttempt)');
 
     _reconnectTimer = Timer(Duration(seconds: delaySec), () async {
       if (!_shouldAutoReconnect || _lastUserConfigJson == null) return;
       try {
-        print('${logTime()}[DesktopVpnBridge] auto-reconnect: attempting start...');
+        logLine('DesktopVpnBridge', 'auto-reconnect: attempting start...');
         await start(_lastUserConfigJson!);
         _reconnectAttempt = 0; // 成功后重置退避计数
-        print('${logTime()}[DesktopVpnBridge] auto-reconnect: success');
+        logLine('DesktopVpnBridge', 'auto-reconnect: success');
       } catch (e) {
-        print('${logTime()}[DesktopVpnBridge] auto-reconnect: start failed: $e');
+        logLine('DesktopVpnBridge', 'auto-reconnect: start failed: $e');
         // start() 失败（内核未能启动）→ 手动调度下一次重试
         if (_shouldAutoReconnect) _scheduleReconnect();
       }
@@ -342,7 +342,7 @@ class DesktopVpnBridge implements VpnBridge {
       try {
         await Process.run('chmod', ['600', file.path]);
       } catch (_) {
-        print('${logTime()}[DesktopVpnBridge] warning: chmod 600 failed for ${file.path}');
+        logLine('DesktopVpnBridge', 'warning: chmod 600 failed for ${file.path}');
       }
     }
     return file.path;