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/tests/e2e/e2e_test.go

@@ -242,7 +242,7 @@ func (s *E2ETestSuite) Router() *gin.Engine {
 func (s *E2ETestSuite) CreateTestAccount(name string) *model.Account {
 	account := &model.Account{
 		Name:     name,
-		Locale:   "en",
+		Locale:   "zh_CN",
 		Timezone: "UTC",
 		Active:   true,
 	}