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/shell/app_shell.dart

@@ -6,6 +6,7 @@ import 'package:intl/intl.dart';
 import 'dart:async';
 import 'package:flutter/foundation.dart' show kIsWeb;
 import '../../core/auth/auth_state.dart';
+import '../../widgets/write_guard.dart';
 import '../../core/config/app_config.dart';
 import '../../core/responsive/responsive.dart';
 import '../../core/theme/app_theme.dart';
@@ -210,7 +211,7 @@ class _AppShellState extends ConsumerState<AppShell> {
                 ),
                 const SizedBox(width: 4),
                 _ShopButton(user: user, version: appVersion),
-                if (ref.watch(isReadonlyProvider)) ...[
+                if (WriteGuard.isReadonly(ref)) ...[
                   const SizedBox(width: 8),
                   Container(
                     padding: