feat(client): App 内在线购买/续费授权（管理员支付宝下单+到账轮询）

- 新增套餐目录 license_plans.dart（标准/高级 × 月付/年付，与官网 checkout 一致）
- license 模型/仓库新增 PurchaseOrder/PurchaseStatusInfo 与下单、查单接口
- 系统设置授权面板：管理员显示「在线购买 / 续费」按钮，弹窗选套餐→
  外部浏览器打开支付宝收银台→3s 轮询到账→自动刷新授权并展示新到期日
- 用户手册（docs.md + user-manual.html）同步补充在线购买入口说明

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JJ1g8XV1YhhmHRzhwWEW7o

client/lib/screens/settings/settings_screen.dart

@@ -32,6 +32,7 @@ import '../../widgets/ds/ds_atoms.dart';
 import '../../widgets/ds/ds_menu.dart';
 import '../../widgets/ds/ds_table.dart';
 import '../../widgets/write_guard.dart';
+import 'purchase_dialog.dart';
 import 'users_screen.dart' show userRoleBadge, userStatusBadge;
 import '../../core/theme/app_fonts.dart';
 import '../../widgets/ds/ds_toast.dart';
@@ -685,6 +686,8 @@ class _LicensePanelState extends ConsumerState<_LicensePanel> {
   Widget build(BuildContext context) {
     final t = context.tokens;
     final lic = ref.watch(licenseProvider).valueOrNull;
+    final role = ref.watch(authStateProvider.select((s) => s.user?.role));
+    final isAdmin = role == 'admin' || role == 'superadmin';
     final expires = lic?.expiresAt != null
         ? DateFormat('yyyy-MM-dd').format(lic!.expiresAt!)
         : '—';
@@ -759,14 +762,21 @@ class _LicensePanelState extends ConsumerState<_LicensePanel> {
           Row(children: [
             Text('没有兑换券？',
                 style: TextStyle(fontSize: AppDims.fsSm, color: t.muted)),
-            InkWell(
-              onTap: () => launchUrl(Uri.parse('${AppInfo.website}/#pricing')),
-              child: Text('购买授权时长 →',
-                  style: TextStyle(
-                      fontSize: AppDims.fsSm,
-                      fontWeight: FontWeight.w600,
-                      color: t.primary)),
-            ),
+            // 在线购买仅管理员（后端 purchase 接口 admin only）；其他角色引导看官网价格页
+            if (isAdmin)
+              DsButton('在线购买 / 续费',
+                  variant: DsBtnVariant.accent,
+                  onPressed: () => showPurchaseDialog(context))
+            else
+              InkWell(
+                onTap: () =>
+                    launchUrl(Uri.parse('${AppInfo.website}/#pricing')),
+                child: Text('查看套餐价格 →',
+                    style: TextStyle(
+                        fontSize: AppDims.fsSm,
+                        fontWeight: FontWeight.w600,
+                        color: t.primary)),
+              ),
           ]),
           const SizedBox(height: 16),
           // 过期降级说明（现有能力保留）