feat(i18n): Add next-intl framework with full bilingual support (EN/ZH)

- Implement next-intl framework for scalable i18n
- Add complete Chinese (Simplified) localization
- Support 400+ translated strings across all pages
- Add language switcher with persistent preference
- Zero breaking changes to existing functionality

Framework additions:
- i18n routing and middleware
- LocaleContext for client-side state
- LanguageSwitcher component
- Translation files (en.json, zh.json)

Translated components:
- Homepage: Hero, features, CTA, navbar
- Auth: Login, register
- Dashboard: Main page, layout
- Connectors: Management, add page (all categories)
- Documents: Upload, manage, filters
- Settings: LLM configs, role assignments
- Onboarding: Add provider, assign roles
- Logs: Task logs viewer

Adding a new language now requires only:
1. Create messages/<locale>.json
2. Add locale to i18n/routing.ts

surfsense_web/components/providers/I18nProvider.tsx

@@ -0,0 +1,19 @@
+'use client';
+
+import { NextIntlClientProvider } from 'next-intl';
+import { useLocaleContext } from '@/contexts/LocaleContext';
+
+/**
+ * I18n Provider component
+ * Wraps NextIntlClientProvider with dynamic locale and messages from LocaleContext
+ */
+export function I18nProvider({ children }: { children: React.ReactNode }) {
+  const { locale, messages } = useLocaleContext();
+
+  return (
+    <NextIntlClientProvider messages={messages} locale={locale}>
+      {children}
+    </NextIntlClientProvider>
+  );
+}
+