feat(frontend): add next-intl bilingual i18n for core paths

Wire next-intl without locale routes, add zh-CN/en catalogs, language
switcher, and migrate layout/auth/settings UI copy. Document i18n rules
in AGENTS.md and keep static export builds working.

frontend/i18n/client.ts

@@ -0,0 +1,72 @@
+'use client';
+
+import {
+  defaultLocale,
+  isAppLocale,
+  localeCookieName,
+  normalizeLocale,
+  type AppLocale,
+} from './config';
+
+const COOKIE_MAX_AGE_SECONDS = 60 * 60 * 24 * 365; // 1 year
+
+function readCookie(name: string): string | null {
+  if (typeof document === 'undefined') return null;
+  const match = document.cookie
+    .split(';')
+    .map((part) => part.trim())
+    .find((part) => part.startsWith(`${name}=`));
+  if (!match) return null;
+  return decodeURIComponent(match.slice(name.length + 1));
+}
+
+export function getClientLocalePreference(): AppLocale | null {
+  const raw = readCookie(localeCookieName);
+  if (isAppLocale(raw)) return raw;
+  if (raw) return normalizeLocale(raw);
+  return null;
+}
+
+export function detectBrowserLocale(): AppLocale {
+  if (typeof navigator === 'undefined') return defaultLocale;
+  const languages =
+    navigator.languages?.length > 0
+      ? navigator.languages
+      : [navigator.language];
+  for (const lang of languages) {
+    const lower = (lang ?? '').toLowerCase();
+    if (lower.startsWith('zh') || lower.startsWith('en')) {
+      return normalizeLocale(lang);
+    }
+  }
+  return defaultLocale;
+}
+
+export function resolveClientLocale(): AppLocale {
+  return getClientLocalePreference() ?? detectBrowserLocale();
+}
+
+export function setLocaleCookie(locale: AppLocale): void {
+  if (typeof document === 'undefined') return;
+  const secure =
+    typeof window !== 'undefined' && window.location.protocol === 'https:'
+      ? '; Secure'
+      : '';
+  document.cookie = `${localeCookieName}=${encodeURIComponent(locale)}; Path=/; Max-Age=${COOKIE_MAX_AGE_SECONDS}; SameSite=Lax${secure}`;
+  try {
+    localStorage.setItem(localeCookieName, locale);
+  } catch {
+    // ignore quota / private mode
+  }
+}
+
+/** Persist locale and reload so server + client messages stay in sync. */
+export function switchLocale(locale: AppLocale): void {
+  setLocaleCookie(locale);
+  if (typeof document !== 'undefined') {
+    document.documentElement.lang = locale;
+  }
+  if (typeof window !== 'undefined') {
+    window.location.reload();
+  }
+}