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/settings/settings_screen.dart

@@ -9,6 +9,7 @@ import 'package:shared_preferences/shared_preferences.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import '../../core/auth/auth_state.dart';
+import '../../widgets/write_guard.dart';
 import '../../core/config/app_config.dart';
 import '../../core/config/app_info.dart';
 import '../../core/responsive/responsive.dart';
@@ -218,12 +219,13 @@ class _SettingsScreenState extends ConsumerState<SettingsScreen> {
           padding: const EdgeInsets.symmetric(horizontal: 12),
           child: Row(
             children: [
-              if (!ref.watch(isReadonlyProvider))
-                ElevatedButton.icon(
+              WriteGuard(
+                child: ElevatedButton.icon(
                   onPressed: () => _showAddUserDialog(context),
                   icon: const Icon(Icons.person_add, size: 16),
                   label: const Text('新增用户'),
                 ),
+              ),
             ],
           ),
         ),
@@ -293,7 +295,7 @@ class _SettingsScreenState extends ConsumerState<SettingsScreen> {
                           )),
                           DataCell(Row(
                             mainAxisSize: MainAxisSize.min,
-                            children: ref.watch(isReadonlyProvider)
+                            children: WriteGuard.isReadonly(ref)
                                 ? const []
                                 : [
                               TextButton(
@@ -457,7 +459,7 @@ class _SettingsScreenState extends ConsumerState<SettingsScreen> {
                     style: const TextStyle(fontSize: 13),
                   ),
                 ),
-                if (!ref.watch(isReadonlyProvider)) ...[
+                if (!WriteGuard.isReadonly(ref)) ...[
                   const SizedBox(width: 8),
                   ElevatedButton(
                     onPressed: _isActivating ? null : _activateLicense,
@@ -619,13 +621,12 @@ class _SettingsScreenState extends ConsumerState<SettingsScreen> {
                               style: const TextStyle(
                                   fontFamily: 'monospace', fontSize: 12))),
                           DataCell(Text('${r.currentNo}')),
-                          DataCell(ref.watch(isReadonlyProvider)
-                              ? const SizedBox()
-                              : TextButton(
+                          DataCell(WriteGuard(
+                              child: TextButton(
                                   onPressed: () =>
                                       _showNumberRuleDialog(context, r),
                                   child: const Text('编辑',
-                                      style: TextStyle(fontSize: 12)))),
+                                      style: TextStyle(fontSize: 12))))),
                         ]))
                     .toList(),
               ),
@@ -804,7 +805,7 @@ class _SettingsScreenState extends ConsumerState<SettingsScreen> {
 
   // ── 数据导入 Tab ──────────────────────────────────────────
   Widget _buildImportTab() {
-    if (ref.watch(isReadonlyProvider)) {
+    if (WriteGuard.isReadonly(ref)) {
       return const Center(
         child: Text('只读账号无导入权限',
             style: TextStyle(color: AppTheme.textSecondary)),