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/views/user/PurchaseSubscriptionView.vue

@@ -0,0 +1,121 @@
+<template>
+  <AppLayout>
+    <div class="purchase-page-layout">
+      <div class="flex items-start justify-between gap-4">
+        <div>
+          <h2 class="text-lg font-semibold text-gray-900 dark:text-white">
+            {{ t('purchase.title') }}
+          </h2>
+          <p class="mt-1 text-sm text-gray-500 dark:text-dark-400">
+            {{ t('purchase.description') }}
+          </p>
+        </div>
+
+        <div class="flex items-center gap-2">
+          <a
+            v-if="isValidUrl"
+            :href="purchaseUrl"
+            target="_blank"
+            rel="noopener noreferrer"
+            class="btn btn-secondary btn-sm"
+          >
+            <Icon name="externalLink" size="sm" class="mr-1.5" :stroke-width="2" />
+            {{ t('purchase.openInNewTab') }}
+          </a>
+        </div>
+      </div>
+
+      <div class="card flex-1 min-h-0 overflow-hidden">
+        <div v-if="loading" class="flex h-full items-center justify-center py-12">
+          <div
+            class="h-8 w-8 animate-spin rounded-full border-2 border-primary-500 border-t-transparent"
+          ></div>
+        </div>
+
+        <div
+          v-else-if="!purchaseEnabled"
+          class="flex h-full items-center justify-center p-10 text-center"
+        >
+          <div class="max-w-md">
+            <div
+              class="mx-auto mb-4 flex h-12 w-12 items-center justify-center rounded-full bg-gray-100 dark:bg-dark-700"
+            >
+              <Icon name="creditCard" size="lg" class="text-gray-400" />
+            </div>
+            <h3 class="text-lg font-semibold text-gray-900 dark:text-white">
+              {{ t('purchase.notEnabledTitle') }}
+            </h3>
+            <p class="mt-2 text-sm text-gray-500 dark:text-dark-400">
+              {{ t('purchase.notEnabledDesc') }}
+            </p>
+          </div>
+        </div>
+
+        <div
+          v-else-if="!isValidUrl"
+          class="flex h-full items-center justify-center p-10 text-center"
+        >
+          <div class="max-w-md">
+            <div
+              class="mx-auto mb-4 flex h-12 w-12 items-center justify-center rounded-full bg-gray-100 dark:bg-dark-700"
+            >
+              <Icon name="link" size="lg" class="text-gray-400" />
+            </div>
+            <h3 class="text-lg font-semibold text-gray-900 dark:text-white">
+              {{ t('purchase.notConfiguredTitle') }}
+            </h3>
+            <p class="mt-2 text-sm text-gray-500 dark:text-dark-400">
+              {{ t('purchase.notConfiguredDesc') }}
+            </p>
+          </div>
+        </div>
+
+        <iframe v-else :src="purchaseUrl" class="h-full w-full border-0" allowfullscreen></iframe>
+      </div>
+    </div>
+  </AppLayout>
+</template>
+
+<script setup lang="ts">
+import { computed, onMounted, ref } from 'vue'
+import { useI18n } from 'vue-i18n'
+import { useAppStore } from '@/stores'
+import AppLayout from '@/components/layout/AppLayout.vue'
+import Icon from '@/components/icons/Icon.vue'
+
+const { t } = useI18n()
+const appStore = useAppStore()
+
+const loading = ref(false)
+
+const purchaseEnabled = computed(() => {
+  return appStore.cachedPublicSettings?.purchase_subscription_enabled ?? false
+})
+
+const purchaseUrl = computed(() => {
+  return (appStore.cachedPublicSettings?.purchase_subscription_url || '').trim()
+})
+
+const isValidUrl = computed(() => {
+  const url = purchaseUrl.value
+  return url.startsWith('http://') || url.startsWith('https://')
+})
+
+onMounted(async () => {
+  if (appStore.publicSettingsLoaded) return
+  loading.value = true
+  try {
+    await appStore.fetchPublicSettings()
+  } finally {
+    loading.value = false
+  }
+})
+</script>
+
+<style scoped>
+.purchase-page-layout {
+  @apply flex flex-col gap-6;
+  height: calc(100vh - 64px - 4rem); /* 减去 header + lg:p-8 的上下padding */
+}
+</style>
+