fix(i18n): normalize locale and prevent undefined translations (#914)

* fix(i18n): guard locale input and add safe translation fallback

* refactor(i18n): isolate locale utils and normalize server cookie decode

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

frontend/src/core/i18n/index.ts

@@ -1,23 +1,11 @@
 export { enUS } from "./locales/en-US";
 export { zhCN } from "./locales/zh-CN";
 export type { Translations } from "./locales/types";
-
-export type Locale = "en-US" | "zh-CN";
-
-// Helper function to detect browser locale
-export function detectLocale(): Locale {
-  if (typeof window === "undefined") {
-    return "en-US";
-  }
-
-  const browserLang =
-    navigator.language ||
-    (navigator as unknown as { userLanguage: string }).userLanguage;
-
-  // Check if browser language is Chinese (zh, zh-CN, zh-TW, etc.)
-  if (browserLang.toLowerCase().startsWith("zh")) {
-    return "zh-CN";
-  }
-
-  return "en-US";
-}
+export {
+  DEFAULT_LOCALE,
+  SUPPORTED_LOCALES,
+  detectLocale,
+  isLocale,
+  normalizeLocale,
+} from "./locale";
+export type { Locale } from "./locale";