feat(i18n): add backend multi-language support with user language preference

- Add go-i18n library for internationalization
- Create i18n package with translation keys and YAML locale files (zh/en)
- Implement i18n middleware for language detection from user settings and Accept-Language header
- Add Language field to UserSetting DTO
- Update API response helpers with i18n support (ApiErrorI18n, ApiSuccessI18n)
- Migrate hardcoded messages in token, redemption, and user controllers
- Add frontend language preference settings component
- Sync language preference across header selector and user settings
- Auto-restore user language preference on login

go.mod

@@ -32,8 +32,10 @@ require (
 	github.com/jinzhu/copier v0.4.0
 	github.com/joho/godotenv v1.5.1
 	github.com/mewkiz/flac v1.0.13
+	github.com/nicksnyder/go-i18n/v2 v2.6.1
 	github.com/pkg/errors v0.9.1
 	github.com/pquerna/otp v1.5.0
+	github.com/samber/hot v0.11.0
 	github.com/samber/lo v1.52.0
 	github.com/shirou/gopsutil v3.21.11+incompatible
 	github.com/shopspring/decimal v1.4.0
@@ -48,7 +50,10 @@ require (
 	golang.org/x/crypto v0.45.0
 	golang.org/x/image v0.23.0
 	golang.org/x/net v0.47.0
-	golang.org/x/sync v0.18.0
+	golang.org/x/sync v0.19.0
+	golang.org/x/sys v0.38.0
+	golang.org/x/text v0.32.0
+	gopkg.in/yaml.v3 v3.0.1
 	gorm.io/driver/mysql v1.4.3
 	gorm.io/driver/postgres v1.5.2
 	gorm.io/gorm v1.25.2
@@ -115,7 +120,6 @@ 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/samber/hot v0.11.0 // 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
@@ -127,10 +131,7 @@ require (
 	github.com/yusufpapurcu/wmi v1.2.3 // indirect
 	golang.org/x/arch v0.21.0 // indirect
 	golang.org/x/exp v0.0.0-20250620022241-b7579e27df2b // indirect
-	golang.org/x/sys v0.38.0 // indirect
-	golang.org/x/text v0.31.0 // indirect
 	google.golang.org/protobuf v1.36.5 // indirect
-	gopkg.in/yaml.v3 v3.0.1 // indirect
 	modernc.org/libc v1.66.10 // indirect
 	modernc.org/mathutil v1.7.1 // indirect
 	modernc.org/memory v1.11.0 // indirect