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/format.ts

@@ -0,0 +1,39 @@
+import type { AppLocale } from './config';
+import { defaultLocale } from './config';
+
+const localeToIntl: Record<AppLocale, string> = {
+  'zh-CN': 'zh-CN',
+  en: 'en-US',
+};
+
+export function formatDateTime(
+  dateStr: string | Date,
+  locale: AppLocale = defaultLocale,
+  timeZone = 'Asia/Shanghai',
+): string {
+  try {
+    const date = typeof dateStr === 'string' ? new Date(dateStr) : dateStr;
+    return new Intl.DateTimeFormat(localeToIntl[locale] ?? locale, {
+      timeZone,
+      year: 'numeric',
+      month: '2-digit',
+      day: '2-digit',
+      hour: '2-digit',
+      minute: '2-digit',
+      hour12: false,
+    }).format(date);
+  } catch {
+    return String(dateStr);
+  }
+}
+
+export function formatNumber(
+  value: number,
+  locale: AppLocale = defaultLocale,
+): string {
+  try {
+    return new Intl.NumberFormat(localeToIntl[locale] ?? locale).format(value);
+  } catch {
+    return String(value);
+  }
+}