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/finance/finance_screen.dart

@@ -11,7 +11,7 @@ import '../../widgets/page_scaffold.dart';
 import '../../providers/connectivity_provider.dart';
 import '../../core/utils/export_util.dart';
 import '../../repositories/finance_repository.dart';
-import '../../core/auth/auth_state.dart';
+import '../../widgets/write_guard.dart';
 
 class FinanceScreen extends ConsumerWidget {
   const FinanceScreen({super.key});
@@ -161,7 +161,7 @@ class _FinanceTabState extends ConsumerState<_FinanceTab> {
           MobileCardField('状态', null, valueWidget: _StatusBadge(r.status)),
         if (r.remark?.isNotEmpty == true) MobileCardField('备注', r.remark),
       ],
-      actions: (canClose && !ref.watch(isReadonlyProvider))
+      actions: (canClose && !WriteGuard.isReadonly(ref))
           ? [
               TextButton(
                 onPressed: () => _closeRecord(r),
@@ -307,7 +307,7 @@ class _FinanceTabState extends ConsumerState<_FinanceTab> {
         case 'actions':
           if ((r.type == 'payable' || r.type == 'receivable') &&
               r.status == 'open' &&
-              !ref.watch(isReadonlyProvider)) {
+              !WriteGuard.isReadonly(ref)) {
             return DataCell(TextButton(
               onPressed: () => _closeRecord(r),
               child: const Text('结清',
@@ -408,13 +408,13 @@ class _FinanceTabState extends ConsumerState<_FinanceTab> {
             },
             toolbar: Row(
               children: [
-                if (addLabel != null && !ref.watch(isReadonlyProvider))
+                if (addLabel != null && !WriteGuard.isReadonly(ref))
                   ElevatedButton.icon(
                     onPressed: _showAddDialog,
                     icon: const Icon(Icons.add, size: 16),
                     label: Text(addLabel),
                   ),
-                if (addLabel != null && !ref.watch(isReadonlyProvider))
+                if (addLabel != null && !WriteGuard.isReadonly(ref))
                   const SizedBox(width: 8),
                 OutlinedButton.icon(
                   onPressed: () {