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/partners/partners_screen.dart

@@ -11,7 +11,7 @@ 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 PartnersScreen extends ConsumerStatefulWidget {
   const PartnersScreen({super.key});
@@ -167,7 +167,7 @@ class _PartnersScreenState extends ConsumerState<PartnersScreen> {
           if (p.phone?.isNotEmpty == true) MobileCardField('电话', p.phone),
           if (p.address?.isNotEmpty == true) MobileCardField('地址', p.address),
         ],
-        actions: ref.watch(isReadonlyProvider)
+        actions: WriteGuard.isReadonly(ref)
             ? const []
             : [
                 TextButton(
@@ -194,7 +194,7 @@ class _PartnersScreenState extends ConsumerState<PartnersScreen> {
       mobileCards: partners.map(partnerCard).toList(),
       toolbar: Row(
         children: [
-          if (!ref.watch(isReadonlyProvider)) ...[
+          if (!WriteGuard.isReadonly(ref)) ...[
             ElevatedButton.icon(
               onPressed: onAdd,
               icon: const Icon(Icons.add, size: 16),
@@ -285,7 +285,7 @@ class _PartnersScreenState extends ConsumerState<PartnersScreen> {
                       )),
                       DataCell(Row(
                         mainAxisSize: MainAxisSize.min,
-                        children: ref.watch(isReadonlyProvider)
+                        children: WriteGuard.isReadonly(ref)
                             ? const []
                             : [
                                 TextButton(