feat(client): 被强制下线时弹对话框告知原因再登出

会话轮询检测到 active=false → 先弹「此设备已被强制下线/需重新登录」对话框(沿用现有
AlertDialog 样式:surface 底/xl 圆角/accent 按钮),用户确认后再 logout 跳登录页,
不再默默跳走。_forcedLogoutHandling 防重复弹。新增 l10n sessionRevoked{Title,Body,Ok}。

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

client/lib/l10n/strings_en.dart

@@ -162,6 +162,12 @@ class StringsEn extends AppText {
   @override
   String get devCancel => 'Cancel';
   @override
+  String get sessionRevokedTitle => 'This device was signed out';
+  @override
+  String get sessionRevokedBody => 'Your account signed this device out from another device. Please sign in again.';
+  @override
+  String get sessionRevokedOk => 'Sign in again';
+  @override
   String get redeemEntry => 'Redeem & buy';
   @override
   String get contactEntry => 'Contact us';