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/app/layout.tsx

@@ -1,5 +1,6 @@
 import type { Metadata } from 'next';
 import { Inter } from 'next/font/google';
+import { getLocale, getMessages, getTranslations } from 'next-intl/server';
 import { Toaster } from '@/components/ui/sonner';
 import { ThemeProvider } from '@/components/layout/theme-provider';
 import { CustomThemeProvider } from '@/lib/theme';
@@ -7,8 +8,10 @@ import { BellRingProvider } from '@/contexts/bell-ring-context';
 import { NotificationSettingsProvider } from '@/contexts/notification-settings-context';
 import { UserProvider } from '@/contexts/user-context';
 import { AppQueryProvider } from '@/components/providers/query-provider';
+import { AppIntlProvider } from '@/components/providers/intl-provider';
 import { SiteTitleUpdater } from '@/components/providers/title-updater';
 import { RobotsMeta } from '@/components/layout/robots-meta';
+import type { AppLocale } from '@/i18n/config';
 import './globals.css';
 
 const inter = Inter({
@@ -17,19 +20,25 @@ const inter = Inter({
   display: 'swap',
 });
 
-export const metadata: Metadata = {
-  title: 'Wavelet Platform',
-  description: '通用二次开发标准范式平台',
-};
+export async function generateMetadata(): Promise<Metadata> {
+  const t = await getTranslations('metadata');
+  return {
+    title: t('title'),
+    description: t('description'),
+  };
+}
 
-export default function RootLayout({
+export default async function RootLayout({
   children,
 }: Readonly<{
   children: React.ReactNode;
 }>) {
+  const locale = (await getLocale()) as AppLocale;
+  const messages = await getMessages();
+
   return (
     <html
-      lang='zh-CN'
+      lang={locale}
       className={`hide-scrollbar font-sans ${inter.variable}`}
       suppressHydrationWarning
     >
@@ -42,16 +51,18 @@ export default function RootLayout({
         >
           <CustomThemeProvider>
             <AppQueryProvider>
-              <SiteTitleUpdater />
-              <RobotsMeta />
-              <UserProvider>
-                <NotificationSettingsProvider>
-                  <BellRingProvider>
-                    {children}
-                    <Toaster position='top-center' />
-                  </BellRingProvider>
-                </NotificationSettingsProvider>
-              </UserProvider>
+              <AppIntlProvider locale={locale} messages={messages}>
+                <SiteTitleUpdater />
+                <RobotsMeta />
+                <UserProvider>
+                  <NotificationSettingsProvider>
+                    <BellRingProvider>
+                      {children}
+                      <Toaster position='top-center' />
+                    </BellRingProvider>
+                  </NotificationSettingsProvider>
+                </UserProvider>
+              </AppIntlProvider>
             </AppQueryProvider>
           </CustomThemeProvider>
         </ThemeProvider>