Fix locale initialization and persistence

frontend/src/core/i18n/context.tsx

@@ -1,8 +1,16 @@
 "use client";
 
-import { createContext, useContext, useState, type ReactNode } from "react";
+import {
+  createContext,
+  useContext,
+  useEffect,
+  useState,
+  type ReactNode,
+} from "react";
 
 import type { Locale } from "@/core/i18n";
+import { getLocaleFromCookie, setLocaleInCookie } from "@/core/i18n/cookies";
+import { detectLocale, normalizeLocale } from "@/core/i18n/locale";
 
 export interface I18nContextType {
   locale: Locale;
@@ -10,6 +18,7 @@ export interface I18nContextType {
 }
 
 export const I18nContext = createContext<I18nContextType | null>(null);
+const LOCALE_STORAGE_KEY = "deerflow_locale";
 
 export function I18nProvider({
   children,
@@ -20,9 +29,48 @@ export function I18nProvider({
 }) {
   const [locale, setLocale] = useState<Locale>(initialLocale);
 
+  useEffect(() => {
+    // Restore locale once at app startup. LocalStorage works even when
+    // third-party cookies are restricted in embedded iframe contexts.
+    const stored =
+      typeof window === "undefined"
+        ? null
+        : window.localStorage.getItem(LOCALE_STORAGE_KEY);
+    const cookieLocale = getLocaleFromCookie();
+
+    if (stored) {
+      const normalizedStored = normalizeLocale(stored);
+      setLocale(normalizedStored);
+      setLocaleInCookie(normalizedStored);
+      return;
+    }
+
+    if (cookieLocale) {
+      const normalizedCookie = normalizeLocale(cookieLocale);
+      setLocale(normalizedCookie);
+      if (cookieLocale !== normalizedCookie) {
+        setLocaleInCookie(normalizedCookie);
+      }
+      if (typeof window !== "undefined") {
+        window.localStorage.setItem(LOCALE_STORAGE_KEY, normalizedCookie);
+      }
+      return;
+    }
+
+    const detected = detectLocale();
+    setLocale(detected);
+    setLocaleInCookie(detected);
+    if (typeof window !== "undefined") {
+      window.localStorage.setItem(LOCALE_STORAGE_KEY, detected);
+    }
+  }, []);
+
   const handleSetLocale = (newLocale: Locale) => {
     setLocale(newLocale);
-    document.cookie = `locale=${newLocale}; path=/; max-age=31536000`;
+    setLocaleInCookie(newLocale);
+    if (typeof window !== "undefined") {
+      window.localStorage.setItem(LOCALE_STORAGE_KEY, newLocale);
+    }
   };
 
   return (