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>

go.mod

@@ -46,6 +46,7 @@ require (
 	github.com/tidwall/gjson v1.18.0
 	github.com/tidwall/sjson v1.2.5
 	github.com/tiktoken-go/tokenizer v0.6.2
+	github.com/waffo-com/waffo-go v1.3.1
 	github.com/yapingcat/gomedia v0.0.0-20240906162731-17feea57090c
 	golang.org/x/crypto v0.45.0
 	golang.org/x/image v0.23.0
@@ -120,7 +121,6 @@ require (
 	github.com/prometheus/procfs v0.15.1 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
 	github.com/samber/go-singleflightx v0.3.2 // indirect
-	github.com/stretchr/objx v0.5.2 // indirect
 	github.com/tidwall/match v1.1.1 // indirect
 	github.com/tidwall/pretty v1.2.0 // indirect
 	github.com/tklauser/go-sysconf v0.3.12 // indirect