feat(client): 设备数超限挡板 DeviceLimitScreen(#16 前端)

登录响应带 device_limit 时,顶层路由把用户挡在「移除设备」页,降到上限内才进主界面。

- auth_api.dart:AuthTokens 解析 device_limit;新增 DeviceLimit/DeviceBrief 模型
- auth_provider.dart:deviceLimitProvider(挡板状态);auth_screen 登录后置入
- main.dart:isLoggedIn 后若 deviceLimit!=null → DeviceLimitScreen(挡 HomeShell)
- screens/device_limit_screen.dart(新):设备列表(本机标注不可删)+ 每台移除
  + 一键「移除最久未用」+ 退出登录;降到上限内自动清挡板放行(复用 devicesProvider.remove)
- l10n:deviceLimitTitle/Desc/RemoveOldest(zh/en);顺带修 devicesSub 文案 5→3 台

analyze 干净;client 全量测试 161 通过。

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

client/lib/services/auth_api.dart

@@ -26,14 +26,63 @@ class AuthApiException implements Exception {
 
 /// 登录 / 注册成功后返回的 JWT 令牌对。
 class AuthTokens {
-  const AuthTokens({required this.accessToken, required this.refreshToken});
+  const AuthTokens({
+    required this.accessToken,
+    required this.refreshToken,
+    this.deviceLimit,
+  });
 
   final String accessToken;
   final String refreshToken;
 
+  /// 非 null 表示登录成功但账户活跃设备数超套餐上限——登录仍生效(token 已发),
+  /// 客户端须先让用户移除设备(降到上限内)才进主界面。见 device_limit 挡板。
+  final DeviceLimit? deviceLimit;
+
   factory AuthTokens.fromJson(Map<String, dynamic> m) => AuthTokens(
         accessToken: m['access_token'] as String? ?? '',
         refreshToken: m['refresh_token'] as String? ?? '',
+        deviceLimit: m['device_limit'] is Map
+            ? DeviceLimit.fromJson(Map<String, dynamic>.from(m['device_limit'] as Map))
+            : null,
+      );
+}
+
+/// 超限信号:当前套餐上限 + 账户活跃设备列表(供「移除设备」页展示)。
+class DeviceLimit {
+  const DeviceLimit({required this.maxDevices, required this.devices});
+
+  final int maxDevices;
+  final List<DeviceBrief> devices;
+
+  factory DeviceLimit.fromJson(Map<String, dynamic> m) => DeviceLimit(
+        maxDevices: (m['max_devices'] as num?)?.toInt() ?? 0,
+        devices: ((m['devices'] as List?) ?? const [])
+            .whereType<Map>()
+            .map((e) => DeviceBrief.fromJson(Map<String, dynamic>.from(e)))
+            .toList(),
+      );
+}
+
+/// 超限页用的精简设备视图。
+class DeviceBrief {
+  const DeviceBrief({
+    required this.uuid,
+    required this.name,
+    required this.platform,
+    this.lastSeen,
+  });
+
+  final String uuid;
+  final String name;
+  final String platform;
+  final String? lastSeen; // RFC 3339 UTC
+
+  factory DeviceBrief.fromJson(Map<String, dynamic> m) => DeviceBrief(
+        uuid: m['uuid'] as String? ?? '',
+        name: m['name'] as String? ?? '',
+        platform: m['platform'] as String? ?? '',
+        lastSeen: m['last_seen'] as String?,
       );
 }