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/app_text.dart

@@ -108,6 +108,10 @@ abstract class AppText {
   String get devForceLogoutConfirm; // 弹窗正文(含设备名占位 %s)
   String get devClearLoginConfirm;
   String get devCancel;
+  // 本设备被其他设备强制下线时的提示框。
+  String get sessionRevokedTitle;
+  String get sessionRevokedBody;
+  String get sessionRevokedOk;
   String get redeemEntry;
   String get contactEntry;
   String get signOut;

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';

client/lib/l10n/strings_zh.dart

@@ -161,6 +161,12 @@ class StringsZh extends AppText {
   @override
   String get devCancel => '取消';
   @override
+  String get sessionRevokedTitle => '此设备已被强制下线';
+  @override
+  String get sessionRevokedBody => '你的账号在其他设备上将此设备强制退出,需要重新登录。';
+  @override
+  String get sessionRevokedOk => '重新登录';
+  @override
   String get redeemEntry => '兑换 & 购买';
   @override
   String get contactEntry => '联系我们';

client/lib/main.dart

@@ -132,6 +132,7 @@ class _RootFlowState extends ConsumerState<RootFlow> {
   bool _autoConnectArmed = false; // 自动连接每进程只尝试一次
   bool _homeLogged = false; // 仅日志:HomeShell 首次显示打一行
   bool _sessionWatchdogArmed = false; // 会话有效性轮询每进程只起一次
+  bool _forcedLogoutHandling = false; // 正在弹「被强制下线」框 + 登出,避免重复
   Timer? _sessionPoll;
 
   @override
@@ -149,19 +150,46 @@ class _RootFlowState extends ConsumerState<RootFlow> {
   }
 
   Future<void> _checkSession() async {
-    if (!mounted || !ref.read(authProvider).isLoggedIn) return;
+    if (!mounted || _forcedLogoutHandling || !ref.read(authProvider).isLoggedIn) return;
     try {
       final deviceId = await ref.read(deviceIdentityProvider).deviceId();
       final active = await ref.read(accountApiProvider).sessionActive(deviceId);
-      if (!active && mounted && ref.read(authProvider).isLoggedIn) {
-        logLine('Session', 'server revoked this device → logout');
-        await ref.read(authProvider.notifier).logout();
-      }
+      if (active || !mounted || !ref.read(authProvider).isLoggedIn) return;
+      // 被其他设备强制下线:先弹对话框告知原因,用户确认后再登出跳登录页。
+      _forcedLogoutHandling = true;
+      logLine('Session', 'server revoked this device → notify + logout');
+      await _showForcedLogoutDialog();
+      if (mounted) await ref.read(authProvider.notifier).logout();
+      _forcedLogoutHandling = false;
     } catch (_) {
       // 网络/鉴权异常不据此登出(fail-safe):只在服务端明确 active=false 才登出。
     }
   }
 
+  Future<void> _showForcedLogoutDialog() async {
+    if (!mounted) return;
+    final c = context.pangolin;
+    final t = ref.read(appTextProvider);
+    await showDialog<void>(
+      context: context,
+      barrierDismissible: false,
+      builder: (ctx) => AlertDialog(
+        backgroundColor: c.surface,
+        shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(PangolinRadius.xl)),
+        title: Text(t.sessionRevokedTitle,
+            style: PangolinText.body.copyWith(color: c.fg1, fontWeight: FontWeight.w700)),
+        content: Text(t.sessionRevokedBody, style: PangolinText.sm.copyWith(color: c.fg2, height: 1.5)),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.pop(ctx),
+            child: Text(t.sessionRevokedOk,
+                style: PangolinText.sm.copyWith(color: c.accent, fontWeight: FontWeight.w700)),
+          ),
+        ],
+      ),
+    );
+  }
+
   // 进入登录态主界面后:开了「自动连接」且当前未连接,就自动连一次。
   // 关键:必须同时等「设置已从持久化加载完(settings.loaded)」与「节点已就绪」——
   // 二者都是异步且完成时刻接近,早读 autoConnect 会撞上还没加载完的默认 false(实测差 1ms)。