feat(client): 只读角色隐藏所有写按钮（WriteGuard 统一控件 + 顶栏只读标识）

- 新增 isReadonlyProvider（role == 'readonly'）+ WriteGuard 控件，统一隐藏写控件
- 入库/出库/库存/商品/基础数据/往来单位/财务/设置 各屏：只读时隐藏
  新增/编辑/删除/审核/提交/驳回/结清/盘点/导入/激活/用户管理 等写操作
- 顶栏显示「只读」标识，让只读用户明确当前权限
- 后端 middleware.ReadOnly() 仍兜底 403，UI 隐藏 + 后端拦截双保险

修正：此前误判为后端漏洞/部署 bug，实为前端写按钮未对只读隐藏。
后端经验证正确拦截只读写操作（线上 stock-in/product-options 均 403）。

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

@@ -11,6 +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';
 
 class FinanceScreen extends ConsumerWidget {
   const FinanceScreen({super.key});
@@ -160,7 +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
+      actions: (canClose && !ref.watch(isReadonlyProvider))
           ? [
               TextButton(
                 onPressed: () => _closeRecord(r),
@@ -304,7 +305,9 @@ class _FinanceTabState extends ConsumerState<_FinanceTab> {
                 style: const TextStyle(fontSize: 12, color: AppTheme.textSecondary)),
           ));
         case 'actions':
-          if ((r.type == 'payable' || r.type == 'receivable') && r.status == 'open') {
+          if ((r.type == 'payable' || r.type == 'receivable') &&
+              r.status == 'open' &&
+              !ref.watch(isReadonlyProvider)) {
             return DataCell(TextButton(
               onPressed: () => _closeRecord(r),
               child: const Text('结清',
@@ -405,13 +408,14 @@ class _FinanceTabState extends ConsumerState<_FinanceTab> {
             },
             toolbar: Row(
               children: [
-                if (addLabel != null)
+                if (addLabel != null && !ref.watch(isReadonlyProvider))
                   ElevatedButton.icon(
                     onPressed: _showAddDialog,
                     icon: const Icon(Icons.add, size: 16),
                     label: Text(addLabel),
                   ),
-                if (addLabel != null) const SizedBox(width: 8),
+                if (addLabel != null && !ref.watch(isReadonlyProvider))
+                  const SizedBox(width: 8),
                 OutlinedButton.icon(
                   onPressed: () {
                     final tabName = widget.typeFilter.isEmpty