refactor(payment): code standards fixes and regression repairs

Backend:
- Split payment_order.go (546→314 lines) into payment_order_lifecycle.go
- Replace magic strings with constants in factory, easypay, webhook handler
- Add rate limit/validity unit constants in payment_order_lifecycle, payment_service
- Fix critical regression: add PaymentEnabled to GetPublicSettings response
- Add missing migration 099_fix_migrated_purchase_menu_label_icon.sql

Frontend:
- Fix StripePopupView.vue: replace `as any` with typed interface, use extractApiErrorMessage
- Fix AdminOrderTable.vue: replace hardcoded column labels with i18n t() calls
- Fix SubscriptionsView.vue: replace hardcoded Today/Tomorrow with i18n
- Extract duplicate statusBadgeClass/canRefund/formatOrderDateTime to orderUtils.ts
- Add missing i18n keys: common.today, common.tomorrow, payment.orders.orderType/actions
- Remove dead PurchaseSubscriptionView.vue (replaced by PaymentView)

frontend/src/components/admin/payment/AdminRefundDialog.vue

@@ -164,6 +164,7 @@ import { reactive, computed, watch } from 'vue'
 import { useI18n } from 'vue-i18n'
 import BaseDialog from '@/components/common/BaseDialog.vue'
 import type { PaymentOrder } from '@/types/payment'
+import { formatOrderDateTime } from '@/components/payment/orderUtils'
 
 const { t } = useI18n()
 
@@ -222,8 +223,7 @@ watch(() => props.show, (val) => {
 })
 
 function formatDateTime(dateStr: string): string {
-  if (!dateStr) return '-'
-  return new Date(dateStr).toLocaleString()
+  return formatOrderDateTime(dateStr)
 }
 
 function handleSubmit() {