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)

backend/internal/handler/setting_handler.go

@@ -57,6 +57,7 @@ func (h *SettingHandler) GetPublicSettings(c *gin.Context) {
 		OIDCOAuthEnabled:                 settings.OIDCOAuthEnabled,
 		OIDCOAuthProviderName:            settings.OIDCOAuthProviderName,
 		BackendModeEnabled:               settings.BackendModeEnabled,
+		PaymentEnabled:                   settings.PaymentEnabled,
 		Version:                          h.version,
 	})
 }