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/locale.ts

@@ -0,0 +1,36 @@
+export const SUPPORTED_LOCALES = ["en-US", "zh-CN"] as const;
+export type Locale = (typeof SUPPORTED_LOCALES)[number];
+export const DEFAULT_LOCALE: Locale = "en-US";
+
+export function isLocale(value: string): value is Locale {
+  return (SUPPORTED_LOCALES as readonly string[]).includes(value);
+}
+
+export function normalizeLocale(locale: string | null | undefined): Locale {
+  if (!locale) {
+    return DEFAULT_LOCALE;
+  }
+
+  if (isLocale(locale)) {
+    return locale;
+  }
+
+  if (locale.toLowerCase().startsWith("zh")) {
+    return "zh-CN";
+  }
+
+  return DEFAULT_LOCALE;
+}
+
+// Helper function to detect browser locale
+export function detectLocale(): Locale {
+  if (typeof window === "undefined") {
+    return DEFAULT_LOCALE;
+  }
+
+  const browserLang =
+    navigator.language ||
+    (navigator as unknown as { userLanguage: string }).userLanguage;
+
+  return normalizeLocale(browserLang);
+}