feat: 连接设备上限 backstop — 兜住"已登录的超限设备"(#16)

登录挡板只拦新登录,已登录的超限会话(如 pro 已连 5 台)永远不被提示 → 限制形同虚设。
补服务端连接卡点:超限账户连接直接拒,兜住已登录设备(下次一连即被拦,无需重登)。

服务端:
- Entitlement 加 MaxDevices(EntitlementForUser 查 p.max_devices;free 默认 1)
- NodeStore.CountActiveDevices(近 30d 活跃)+ SQLNodeStore/mock 实现
- ConnectNode:activeCount > MaxDevices → 403 {code:DEVICE_LIMIT_EXCEEDED, max_devices}

客户端:
- ConnectApiException 解析 code/max_devices(结构化错误体)
- _connect 遇 DEVICE_LIMIT_EXCEEDED → 置 deviceLimitProvider → 顶层路由弹「移除设备」页
  (设备列表走 devicesProvider),移除到上限内自动放行

后端 go test(store/nodes/httpapi/devices)全过;前端 analyze 干净 + 66 测试过。

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

client/lib/services/connect_api.dart

@@ -16,14 +16,22 @@ class ConnectApiException implements Exception {
     required this.statusCode,
     required this.messageZh,
     required this.messageEn,
+    this.code,
+    this.maxDevices,
   });
 
   final int statusCode;
   final String messageZh;
   final String messageEn;
 
+  /// 服务端错误码(如 DEVICE_LIMIT_EXCEEDED);网络/解析错误时为 null。
+  final String? code;
+
+  /// DEVICE_LIMIT_EXCEEDED 时服务端下发的套餐设备上限。
+  final int? maxDevices;
+
   @override
-  String toString() => 'ConnectApiException($statusCode): $messageZh';
+  String toString() => 'ConnectApiException($statusCode${code != null ? '/$code' : ''}): $messageZh';
 }
 
 /// [ConnectApi] 封装 POST /v1/nodes/:id/connect 调用。
@@ -76,10 +84,25 @@ class ConnectApi {
     }
 
     if (response.statusCode != 200) {
+      // 解析结构化错误体(code/message/max_devices),失败则回退通用文案。
+      String? code;
+      int? maxDevices;
+      String? zh, en;
+      try {
+        final m = jsonDecode(response.body);
+        if (m is Map) {
+          code = m['code'] as String?;
+          maxDevices = (m['max_devices'] as num?)?.toInt();
+          zh = m['message_zh'] as String?;
+          en = m['message_en'] as String?;
+        }
+      } catch (_) {/* 非 JSON,用通用文案 */}
       throw ConnectApiException(
         statusCode: response.statusCode,
-        messageZh: '节点连接失败 (HTTP ${response.statusCode})',
-        messageEn: 'Connect failed (HTTP ${response.statusCode})',
+        code: code,
+        maxDevices: maxDevices,
+        messageZh: zh ?? '节点连接失败 (HTTP ${response.statusCode})',
+        messageEn: en ?? 'Connect failed (HTTP ${response.statusCode})',
       );
     }
 

client/lib/state/connection_provider.dart

@@ -16,6 +16,7 @@ import '../bridge/vpn_bridge_provider.dart';
 import '../l10n/app_text.dart';
 import '../models/node.dart';
 import '../services/api_config.dart';
+import '../services/auth_api.dart';
 import '../services/connect_api.dart';
 import '../services/device_identity.dart';
 import 'app_providers.dart';
@@ -193,6 +194,14 @@ class ConnectionController extends StateNotifier<ConnectionState> {
       // bridge.start() 不阻塞至连接建立；on 状态由 statusStream 回调驱动。
       await _bridge.start(configJson);
     } on ConnectApiException catch (e) {
+      // 设备数超限(backstop):弹「移除设备」页(顶层路由据 deviceLimitProvider 切屏),
+      // 兜住"已登录但超限"的设备——它们一连就被拦、被提示,无需重新登录。连接回 off。
+      if (e.code == 'DEVICE_LIMIT_EXCEEDED') {
+        _ref.read(deviceLimitProvider.notifier).state =
+            DeviceLimit(maxDevices: e.maxDevices ?? 0, devices: const []);
+        if (mounted) state = const ConnectionState(phase: VpnPhase.off);
+        return;
+      }
       // 把后端/网络错误冒泡到 UI(原静默回 off,用户不知所以)。
       if (mounted) state = ConnectionState(phase: VpnPhase.off, error: zh ? e.messageZh : e.messageEn);
     } catch (e) {