feat: save locale in cookies

frontend/src/core/i18n/context.tsx

@@ -0,0 +1,41 @@
+"use client";
+
+import { createContext, useContext, useState, type ReactNode } from "react";
+
+import type { Locale } from "@/core/i18n";
+
+export interface I18nContextType {
+  locale: Locale;
+  setLocale: (locale: Locale) => void;
+}
+
+export const I18nContext = createContext<I18nContextType | null>(null);
+
+export function I18nProvider({
+  children,
+  initialLocale,
+}: {
+  children: ReactNode;
+  initialLocale: Locale;
+}) {
+  const [locale, setLocale] = useState<Locale>(initialLocale);
+
+  const handleSetLocale = (newLocale: Locale) => {
+    setLocale(newLocale);
+    document.cookie = `locale=${newLocale}; path=/; max-age=31536000`;
+  };
+
+  return (
+    <I18nContext.Provider value={{ locale, setLocale: handleSetLocale }}>
+      {children}
+    </I18nContext.Provider>
+  );
+}
+
+export function useI18nContext() {
+  const context = useContext(I18nContext);
+  if (!context) {
+    throw new Error("useI18n must be used within I18nProvider");
+  }
+  return context;
+}