feat(waffo): Waffo payment gateway integration with configurable methods

- Add Waffo payment SDK integration (waffo-go v1.3.1)
- Backend: webhook handler, pay endpoint, order lock race-condition fix
- Settings: full Waffo config (API keys, sandbox/prod, currency, pay methods)
- Frontend: Waffo payment buttons in topup page, admin settings panel
- i18n: Waffo-related translations for en/fr/ja/ru/vi/zh-TW

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

constant/waffo_pay_method.go

@@ -0,0 +1,16 @@
+package constant
+
+// WaffoPayMethod defines the display and API parameter mapping for Waffo payment methods.
+type WaffoPayMethod struct {
+	Name          string `json:"name"`            // Frontend display name
+	Icon          string `json:"icon"`            // Frontend icon identifier: credit-card, apple, google
+	PayMethodType string `json:"payMethodType"` // Waffo API PayMethodType, can be comma-separated
+	PayMethodName string `json:"payMethodName"` // Waffo API PayMethodName, empty means auto-select by Waffo checkout
+}
+
+// DefaultWaffoPayMethods is the default list of supported payment methods.
+var DefaultWaffoPayMethods = []WaffoPayMethod{
+	{Name: "Card", Icon: "/pay-card.png", PayMethodType: "CREDITCARD,DEBITCARD", PayMethodName: ""},
+	{Name: "Apple Pay", Icon: "/pay-apple.png", PayMethodType: "APPLEPAY", PayMethodName: "APPLEPAY"},
+	{Name: "Google Pay", Icon: "/pay-google.png", PayMethodType: "GOOGLEPAY", PayMethodName: "GOOGLEPAY"},
+}