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/config.ts

@@ -0,0 +1,67 @@
+export const locales = ['zh-CN', 'en'] as const;
+
+export type AppLocale = (typeof locales)[number];
+
+export const defaultLocale: AppLocale = 'zh-CN';
+
+export const localeCookieName = 'NEXT_LOCALE';
+
+export const localeLabels: Record<AppLocale, string> = {
+  'zh-CN': '中文',
+  en: 'English',
+};
+
+export function isAppLocale(
+  value: string | undefined | null,
+): value is AppLocale {
+  return value === 'zh-CN' || value === 'en';
+}
+
+/** Normalize BCP 47 / browser tags into a supported app locale. */
+export function normalizeLocale(input?: string | null): AppLocale {
+  if (!input) return defaultLocale;
+  const tag = input.trim().replaceAll('_', '-');
+  if (!tag) return defaultLocale;
+
+  const lower = tag.toLowerCase();
+  if (lower === 'zh-cn' || lower.startsWith('zh-hans') || lower === 'zh') {
+    return 'zh-CN';
+  }
+  if (lower.startsWith('zh')) {
+    // zh-TW / zh-Hant etc. map to zh-CN until a dedicated locale exists
+    return 'zh-CN';
+  }
+  if (lower === 'en' || lower.startsWith('en-')) {
+    return 'en';
+  }
+  return defaultLocale;
+}
+
+/** Pick the best locale from an Accept-Language style list. */
+export function pickLocaleFromAcceptLanguage(
+  header?: string | null,
+): AppLocale {
+  if (!header) return defaultLocale;
+
+  const candidates = header
+    .split(',')
+    .map((part) => {
+      const [tag, ...params] = part.trim().split(';');
+      const qParam = params.find((p) => p.trim().startsWith('q='));
+      const q = qParam ? Number.parseFloat(qParam.split('=')[1] ?? '1') : 1;
+      return { tag: tag?.trim() ?? '', q: Number.isFinite(q) ? q : 0 };
+    })
+    .filter((c) => c.tag)
+    .sort((a, b) => b.q - a.q);
+
+  for (const candidate of candidates) {
+    const normalized = normalizeLocale(candidate.tag);
+    // Only accept if the raw tag roughly matches a supported family
+    const lower = candidate.tag.toLowerCase();
+    if (lower.startsWith('zh') || lower.startsWith('en')) {
+      return normalized;
+    }
+  }
+
+  return defaultLocale;
+}