fix(client): 只读模式下允许激活兑换券（自救出口）

授权到期进入 readonly/locked 相位后，「兑换续期」按钮被全局
WriteGuard 误锁（置灰/隐藏），用户无法激活新授权码自救。后端
/license/* 本就豁免 LicenseGuard，去掉该按钮外层 WriteGuard 包裹；
role=readonly 仍由后端 ReadOnly() 403 兜底。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bupi8Kdqkfx2N5acFsHTx5

client/lib/screens/settings/license_panel.dart

@@ -18,7 +18,6 @@ import '../../core/utils/clock.dart';
 import '../../providers/license_provider.dart';
 import '../../widgets/ds/ds_atoms.dart';
 import '../../widgets/ds/ds_toast.dart';
-import '../../widgets/write_guard.dart';
 import 'settings_card.dart';
 
 class LicensePanel extends ConsumerStatefulWidget {
@@ -123,11 +122,12 @@ class _LicensePanelState extends ConsumerState<LicensePanel> {
                   ),
                 ),
                 const SizedBox(width: 10),
-                WriteGuard(
-                  child: DsButton(_redeeming ? '兑换中…' : '兑换续期',
-                      variant: DsBtnVariant.primary,
-                      onPressed: _redeeming ? null : _redeem),
-                ),
+                // 激活/兑换是过期后的「自救出口」：后端 /license/* 豁免 LicenseGuard，
+                // 故此按钮不受授权相位(readonly/locked)只读锁约束，绝不能包 WriteGuard；
+                // role=readonly 用户由后端 ReadOnly() 403 + toast 兜底。
+                DsButton(_redeeming ? '兑换中…' : '兑换续期',
+                    variant: DsBtnVariant.primary,
+                    onPressed: _redeeming ? null : _redeem),
               ],
             ),
           ),