feat(client): 实时授权状态与会话失效处理

- 心跳读取 /auth/ping 回带的授权概况，直接刷新横幅/状态栏/只读门禁，省去单独轮询 /license/info
- 账号被停用/删除（401 USER_DISABLED）即强制重新登录
- 令牌持久化经串行队列 + 会话代号守卫，杜绝续期写入与登出交叉把失效 token 写回
- 写操作门禁（write_guard）+ 授权文案（license_copy）按 grace/readonly/locked 分阶段降级

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

client/lib/models/license.dart

@@ -50,6 +50,13 @@ class LicenseInfo {
     return diff < 0 ? 0 : diff;
   }
 
+  /// 已过期天数（未过期或永久授权返回 0）。
+  int get daysExpired {
+    if (expiresAt == null) return 0;
+    final diff = DateTime.now().difference(expiresAt!).inDays;
+    return diff < 0 ? 0 : diff;
+  }
+
   bool get isReadOnlyPhase => phase == 'readonly' || phase == 'locked';
   bool get isLockedPhase => phase == 'locked';
   bool get needsAttention => phase == 'grace' || phase == 'readonly' || phase == 'locked';