feat(payment): 订阅套餐支持币种标注，区分 $ 符号对应币种

后台配置与用户端展示的套餐价格统一使用 $ 符号，而实际扣款币种随
支付渠道配置漂移（如 Stripe 配置 NZD 时按新西兰元扣款），用户易
误认为美元。本次为套餐新增可选的展示性币种标注：

- SubscriptionPlan 新增 currency 字段（ISO 4217 三字母码，默认空）
- 空值不展示任何标注，存量套餐行为完全不变
- 非空值复用 payment.NormalizePaymentCurrency 校验并规范化为大写
- 后台套餐编辑弹窗新增可选币种输入，列表价格列展示币种小字
- 用户端套餐卡片价格与划线原价旁展示币种小字
- 纯展示性质，不影响任何支付/扣款逻辑

Closes #4315

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/views/admin/orders/AdminPaymentPlansView.vue

@@ -30,6 +30,7 @@
         <template #cell-price="{ value, row }">
           <div class="text-sm">
             <span class="font-medium text-gray-900 dark:text-white">${{ (value ?? 0).toFixed(2) }}</span>
+            <span v-if="row.currency" class="ml-1 text-xs text-gray-400">{{ row.currency }}</span>
             <span v-if="row.original_price" class="ml-1 text-xs text-gray-400 line-through">${{ row.original_price.toFixed(2) }}</span>
           </div>
         </template>

frontend/src/views/admin/orders/PlanEditDialog.vue

@@ -53,6 +53,11 @@
       </div>
       <div class="grid grid-cols-2 gap-4">
         <div><label class="input-label">{{ t('payment.admin.sortOrder') }}</label><input v-model.number="planForm.sort_order" type="number" min="0" class="input" /></div>
+        <div>
+          <label class="input-label">{{ t('payment.admin.currency') }}</label>
+          <input v-model="planForm.currency" type="text" maxlength="3" class="input uppercase" :placeholder="t('payment.admin.currencyPlaceholder')" />
+          <p class="mt-1 text-xs text-gray-500 dark:text-gray-400">{{ t('payment.admin.currencyHint') }}</p>
+        </div>
       </div>
       <div>
         <label class="input-label">{{ t('payment.admin.features') }}</label>
@@ -117,7 +122,7 @@ const { t } = useI18n()
 const appStore = useAppStore()
 
 const saving = ref(false)
-const planForm = reactive({ name: '', group_id: null as number | null, description: '', price: 0, original_price: 0, validity_days: 30, validity_unit: 'days', sort_order: 0, for_sale: true })
+const planForm = reactive({ name: '', group_id: null as number | null, description: '', price: 0, original_price: 0, currency: '', validity_days: 30, validity_unit: 'days', sort_order: 0, for_sale: true })
 const planFeaturesText = ref('')
 
 const validityUnitOptions = computed(() => [
@@ -170,10 +175,10 @@ const subscriptionCnyPreview = computed(() => {
 watch(() => props.show, (visible) => {
   if (!visible) return
   if (props.plan) {
-    Object.assign(planForm, { name: props.plan.name, group_id: props.plan.group_id, description: props.plan.description, price: props.plan.price, original_price: props.plan.original_price || 0, validity_days: props.plan.validity_days, validity_unit: props.plan.validity_unit || 'days', sort_order: props.plan.sort_order || 0, for_sale: props.plan.for_sale })
+    Object.assign(planForm, { name: props.plan.name, group_id: props.plan.group_id, description: props.plan.description, price: props.plan.price, original_price: props.plan.original_price || 0, currency: props.plan.currency || '', validity_days: props.plan.validity_days, validity_unit: props.plan.validity_unit || 'days', sort_order: props.plan.sort_order || 0, for_sale: props.plan.for_sale })
     planFeaturesText.value = (props.plan.features || []).join('\n')
   } else {
-    Object.assign(planForm, { name: '', group_id: null, description: '', price: 0, original_price: 0, validity_days: 30, validity_unit: 'days', sort_order: 0, for_sale: true })
+    Object.assign(planForm, { name: '', group_id: null, description: '', price: 0, original_price: 0, currency: '', validity_days: 30, validity_unit: 'days', sort_order: 0, for_sale: true })
     planFeaturesText.value = ''
   }
 })
@@ -187,6 +192,7 @@ function buildPlanPayload() {
     description: planForm.description,
     price: planForm.price,
     original_price: planForm.original_price || 0,
+    currency: planForm.currency.trim().toUpperCase(),
     validity_days: planForm.validity_days,
     validity_unit: planForm.validity_unit,
     sort_order: planForm.sort_order,