refactor(client): 只读判断统一走 WriteGuard 控件

将各屏内联的 ref.watch(isReadonlyProvider) 只读判断统一改为
WriteGuard(child:) 或 WriteGuard.isReadonly(ref)，全项目只读逻辑
单一入口（仅 write_guard.dart 内部保留 provider 读取）。行为不变。

121 测试通过，analyze 无 error。

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

client/lib/screens/stock_out/stock_out_list_screen.dart

@@ -20,7 +20,7 @@ import '../../providers/inventory_provider.dart';
 import '../../providers/tab_state_provider.dart';
 import '../../providers/product_provider.dart';
 import '../../providers/finance_provider.dart' show financeRepositoryProvider;
-import '../../core/auth/auth_state.dart';
+import '../../widgets/write_guard.dart';
 
 class StockOutListScreen extends ConsumerStatefulWidget {
   const StockOutListScreen({super.key});
@@ -287,7 +287,7 @@ class _StockOutListScreenState extends ConsumerState<StockOutListScreen> {
           ref.read(stockOutListProvider.notifier).setPageSize(s),
       toolbar: Row(
         children: [
-          if (showNewButton && !ref.watch(isReadonlyProvider))
+          if (showNewButton && !WriteGuard.isReadonly(ref))
             ElevatedButton.icon(
               onPressed: () => context.go('/stock-out/new'),
               icon: const Icon(Icons.add, size: 16),
@@ -351,7 +351,7 @@ class _StockOutListScreenState extends ConsumerState<StockOutListScreen> {
 
   /// 操作按钮列表，表格与移动端卡片共用。
   List<Widget> _orderActions(BuildContext context, StockOutOrder o) {
-    final readonly = ref.watch(isReadonlyProvider);
+    final readonly = WriteGuard.isReadonly(ref);
     return [
       TextButton(
         onPressed: () => _showDetail(context, o.id),