feat(purchase): 增加购买订阅 iframe 页面与配置

- 新增 /purchase 页面（iframe + 新窗口兜底）

- 管理员系统设置可配置开关与URL

- 非 simple mode 才在侧边栏展示入口

frontend/src/i18n/locales/en.ts

@@ -206,6 +206,7 @@ export default {
     logout: 'Logout',
     github: 'GitHub',
     mySubscriptions: 'My Subscriptions',
+    buySubscription: 'Purchase Subscription',
     docs: 'Docs'
   },
 
@@ -2894,6 +2895,17 @@ export default {
         hideCcsImportButton: 'Hide CCS Import Button',
         hideCcsImportButtonHint: 'When enabled, the "Import to CCS" button will be hidden on the API Keys page'
       },
+      purchase: {
+        title: 'Purchase Page',
+        description: 'Show a "Purchase Subscription" entry in the sidebar and open the configured URL in an iframe',
+        enabled: 'Show Purchase Entry',
+        enabledHint: 'Only shown in standard mode (not simple mode)',
+        url: 'Purchase URL',
+        urlPlaceholder: 'https://example.com/purchase',
+        urlHint: 'Must be an absolute http(s) URL',
+        iframeWarning:
+          '⚠️ iframe note: Some websites block embedding via X-Frame-Options or CSP (frame-ancestors). If the page is blank, provide an "Open in new tab" alternative.'
+      },
       smtp: {
         title: 'SMTP Settings',
         description: 'Configure email sending for verification codes',
@@ -3039,6 +3051,18 @@ export default {
     retry: 'Retry'
   },
 
+  // Purchase Subscription Page
+  purchase: {
+    title: 'Purchase Subscription',
+    description: 'Purchase a subscription via the embedded page',
+    openInNewTab: 'Open in new tab',
+    notEnabledTitle: 'Feature not enabled',
+    notEnabledDesc: 'The administrator has not enabled the purchase page. Please contact admin.',
+    notConfiguredTitle: 'Purchase URL not configured',
+    notConfiguredDesc:
+      'The administrator enabled the entry but has not configured a purchase URL. Please contact admin.'
+  },
+
   // User Subscriptions Page
   userSubscriptions: {
     title: 'My Subscriptions',