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/pkg/testutil/fixtures.go

@@ -17,7 +17,7 @@ import (
 func NewAccountFixture(name string) *model.Account {
 	return &model.Account{
 		Name:     name,
-		Locale:   "en",
+		Locale:   "zh_CN",
 		Timezone: "UTC",
 		Active:   true,
 	}

backend/pkg/testutil/helpers.go

@@ -166,7 +166,7 @@ func AssertTimestampOrder(t *testing.T, createdAt, updatedAt time.Time) {
 // SeedAccount creates and persists a test account.
 func SeedAccount(t *testing.T, db *gorm.DB, name string) *model.Account {
 	t.Helper()
-	acc := &model.Account{Name: name, Locale: "en", Timezone: "UTC", Active: true}
+	acc := &model.Account{Name: name, Locale: "zh_CN", Timezone: "UTC", Active: true}
 	err := db.Create(acc).Error
 	assert.NoError(t, err, "failed to seed account")
 	return acc