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/l10n/strings_en.dart

@@ -130,9 +130,15 @@ class StringsEn extends AppText {
   @override
   String get accChange => 'Change';
   @override
+  String get deviceLimitTitle => 'Device limit reached';
+  @override
+  String get deviceLimitDesc => 'Your plan allows up to %s devices. Remove one to continue.';
+  @override
+  String get deviceLimitRemoveOldest => 'Remove least recently used';
+  @override
   String get myDevices => 'My devices';
   @override
-  String get devicesSub => 'Up to 5 devices on PRO';
+  String get devicesSub => 'Up to 3 devices on PRO';
   @override
   String get thisDevice => 'This device';
   @override