fix(client): 连接时长按墙上时钟算,切后台/锁屏不漏计

iOS 锁屏/切后台后连接页时长少算:elapsed 原来靠每秒 +1 的 Timer 累加,后台
isolate 被挂起、Timer 停跳 → 后台那段时长漏掉。改为记 _connectedAt 起点,
elapsed = now - _connectedAt(墙上时钟),与 timer 是否跳无关;回前台(resumed)
立即 _refreshElapsed 补上。_connectedAt 用 ??= 保留同一会话起点,断开才清。

测试:新增「切后台 300s 时长不漏计」回归(前台 10s + 后台 300s = 310s);全 8 例过。

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

client/lib/state/connection_provider.dart

@@ -101,7 +101,10 @@ class ConnectionController extends StateNotifier<ConnectionState> {
     _lifecycle = AppLifecycleListener(
       onStateChange: (s) {
         if (s == AppLifecycleState.resumed) {
-          if (state.phase == VpnPhase.on && _watchdog == null) _startWatchdog();
+          if (state.phase == VpnPhase.on) {
+            if (_watchdog == null) _startWatchdog();
+            _refreshElapsed(); // 回前台按墙上时钟补上后台漏计的时长
+          }
         } else if (s == AppLifecycleState.paused || s == AppLifecycleState.hidden) {
           _stopWatchdog();
         }
@@ -115,6 +118,9 @@ class ConnectionController extends StateNotifier<ConnectionState> {
   StreamSubscription<VpnStatsEvent>? _statsSub;
   ProviderSubscription<AuthState>? _authSub;
   Timer? _elapsed;
+  // 本次连接的起始时刻;连接时长按「now - _connectedAt」墙上时钟算,而非每秒 +1 累加 ——
+  // 否则 app 切后台/锁屏 isolate 被挂起、Timer 停跳,后台那段时长会漏计(iOS 尤甚)。
+  DateTime? _connectedAt;
   Timer? _watchdog;
   AppLifecycleListener? _lifecycle;
   bool _probing = false;
@@ -377,16 +383,23 @@ class ConnectionController extends StateNotifier<ConnectionState> {
 
   void _startElapsed() {
     _elapsed?.cancel();
-    _elapsed = Timer.periodic(const Duration(seconds: 1), (_) {
-      if (mounted && state.phase == VpnPhase.on) {
-        state = state.copyWith(elapsed: state.elapsed + const Duration(seconds: 1));
-      }
-    });
+    _connectedAt ??= _now();               // 同一会话保留起点(on 被重复上报也不重置);断开时清空
+    _refreshElapsed();                     // 立即刷新,避免等 1s
+    _elapsed = Timer.periodic(const Duration(seconds: 1), (_) => _refreshElapsed());
+  }
+
+  /// 按墙上时钟把 elapsed 刷成 now - _connectedAt(切后台回来也准)。
+  void _refreshElapsed() {
+    final at = _connectedAt;
+    if (mounted && state.phase == VpnPhase.on && at != null) {
+      state = state.copyWith(elapsed: _now().difference(at));
+    }
   }
 
   void _stopElapsed() {
     _elapsed?.cancel();
     _elapsed = null;
+    _connectedAt = null;
   }
 
   @override