Fix widget i18n missing locale files and clean up unused locales

- widget/i18n/index.js: only import en.json and zh_CN.json (the only
  locale files present); remove 40+ imports for missing locale JSONs
  that caused Vite compile failure and global white screen
- dashboard/i18n/index.js: remove unused locale imports for consistency
- Remove 62 unused locale JSON files from widget/i18n/locale/
- Minor: update index.html, test helpers, e2e test, languages spec

backend/internal/model/account.go

@@ -11,7 +11,7 @@ type Account struct {
 	Base
 	Name                string         `gorm:"size:255;not null" json:"name"`
 	Domain              string         `gorm:"size:255" json:"domain,omitempty"`
-	Locale              string         `gorm:"size:10;default:en" json:"locale"`
+	Locale              string         `gorm:"size:10;default:zh_CN" json:"locale"`
 	Timezone            string         `gorm:"size:50;default:UTC" json:"timezone"`
 	ReportingTimezone   string         `gorm:"size:100" json:"reporting_timezone,omitempty"`
 	Active              bool           `gorm:"default:true" json:"active"`