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/products/products_screen.dart

@@ -10,7 +10,7 @@ import '../../widgets/data_table_card.dart';
 import '../../widgets/mobile_list_card.dart';
 import '../../widgets/page_scaffold.dart';
 import '../../core/utils/export_util.dart';
-import '../../core/auth/auth_state.dart';
+import '../../widgets/write_guard.dart';
 
 class ProductsScreen extends ConsumerStatefulWidget {
   const ProductsScreen({super.key});
@@ -355,10 +355,9 @@ class _ProductsScreenState extends ConsumerState<ProductsScreen> {
     required VoidCallback onAdd,
     required VoidCallback onExport,
   }) {
-    final readonly = ref.watch(isReadonlyProvider);
     return Row(
       children: [
-        if (!readonly) ...[
+        if (!WriteGuard.isReadonly(ref)) ...[
           ElevatedButton.icon(
             onPressed: onAdd,
             icon: const Icon(Icons.add, size: 16),
@@ -420,7 +419,7 @@ class _ProductsScreenState extends ConsumerState<ProductsScreen> {
           MobileCardField('单品数量', '$quantity'),
         if (remark?.isNotEmpty == true) MobileCardField('备注', remark),
       ],
-      actions: ref.watch(isReadonlyProvider)
+      actions: WriteGuard.isReadonly(ref)
           ? const []
           : [
               TextButton(
@@ -437,19 +436,20 @@ class _ProductsScreenState extends ConsumerState<ProductsScreen> {
   }
 
   Widget _actionButtons({required VoidCallback onEdit, required VoidCallback onDelete}) {
-    if (ref.watch(isReadonlyProvider)) return const SizedBox.shrink();
-    return Row(
-      mainAxisSize: MainAxisSize.min,
-      children: [
-        TextButton(
-          onPressed: onEdit,
-          child: const Text('编辑', style: TextStyle(fontSize: 12)),
-        ),
-        TextButton(
-          onPressed: onDelete,
-          child: const Text('删除', style: TextStyle(fontSize: 12, color: AppTheme.danger)),
-        ),
-      ],
+    return WriteGuard(
+      child: Row(
+        mainAxisSize: MainAxisSize.min,
+        children: [
+          TextButton(
+            onPressed: onEdit,
+            child: const Text('编辑', style: TextStyle(fontSize: 12)),
+          ),
+          TextButton(
+            onPressed: onDelete,
+            child: const Text('删除', style: TextStyle(fontSize: 12, color: AppTheme.danger)),
+          ),
+        ],
+      ),
     );
   }