feat: save locale in cookies

frontend/src/core/i18n/cookies.ts

@@ -0,0 +1,52 @@
+/**
+ * Cookie utilities for locale management
+ * Works on both client and server side
+ */
+
+const LOCALE_COOKIE_NAME = "locale";
+
+/**
+ * Get locale from cookie (client-side)
+ */
+export function getLocaleFromCookie(): string | null {
+  if (typeof document === "undefined") {
+    return null;
+  }
+
+  const cookies = document.cookie.split(";");
+  for (const cookie of cookies) {
+    const [name, value] = cookie.trim().split("=");
+    if (name === LOCALE_COOKIE_NAME) {
+      return decodeURIComponent(value ?? "");
+    }
+  }
+  return null;
+}
+
+/**
+ * Set locale in cookie (client-side)
+ */
+export function setLocaleInCookie(locale: string): void {
+  if (typeof document === "undefined") {
+    return;
+  }
+
+  // Set cookie with 1 year expiration
+  const maxAge = 365 * 24 * 60 * 60; // 1 year in seconds
+  document.cookie = `${LOCALE_COOKIE_NAME}=${encodeURIComponent(locale)}; max-age=${maxAge}; path=/; SameSite=Lax`;
+}
+
+/**
+ * Get locale from cookie (server-side)
+ * Use this in server components or API routes
+ */
+export async function getLocaleFromCookieServer(): Promise<string | null> {
+  try {
+    const { cookies } = await import("next/headers");
+    const cookieStore = await cookies();
+    return cookieStore.get(LOCALE_COOKIE_NAME)?.value ?? null;
+  } catch {
+    // Fallback if cookies() is not available (e.g., in middleware)
+    return null;
+  }
+}