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/screens/products/products_screen.dart

@@ -358,10 +358,12 @@ class _ProductsScreenState extends ConsumerState<ProductsScreen> {
     return Row(
       children: [
         if (!WriteGuard.isReadonly(ref)) ...[
-          ElevatedButton.icon(
-            onPressed: onAdd,
-            icon: const Icon(Icons.add, size: 16),
-            label: const Text('新建'),
+          WriteGuard(
+            child: ElevatedButton.icon(
+              onPressed: onAdd,
+              icon: const Icon(Icons.add, size: 16),
+              label: const Text('新建'),
+            ),
           ),
           const SizedBox(width: 8),
         ],
@@ -422,14 +424,18 @@ class _ProductsScreenState extends ConsumerState<ProductsScreen> {
       actions: WriteGuard.isReadonly(ref)
           ? const []
           : [
-              TextButton(
-                onPressed: onEdit,
-                child: const Text('编辑', style: TextStyle(fontSize: 13)),
+              WriteGuard(
+                child: TextButton(
+                  onPressed: onEdit,
+                  child: const Text('编辑', style: TextStyle(fontSize: 13)),
+                ),
               ),
-              TextButton(
-                onPressed: onDelete,
-                child: const Text('删除',
-                    style: TextStyle(fontSize: 13, color: AppTheme.danger)),
+              WriteGuard(
+                child: TextButton(
+                  onPressed: onDelete,
+                  child: const Text('删除',
+                      style: TextStyle(fontSize: 13, color: AppTheme.danger)),
+                ),
               ),
             ],
     );