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/contexts/LocaleContext.tsx

@@ -0,0 +1,70 @@
+'use client';
+
+import React, { createContext, useContext, useState, useEffect } from 'react';
+import enMessages from '../messages/en.json';
+import zhMessages from '../messages/zh.json';
+
+type Locale = 'en' | 'zh';
+
+interface LocaleContextType {
+  locale: Locale;
+  messages: typeof enMessages;
+  setLocale: (locale: Locale) => void;
+}
+
+const LocaleContext = createContext<LocaleContextType | undefined>(undefined);
+
+const LOCALE_STORAGE_KEY = 'surfsense-locale';
+
+export function LocaleProvider({ children }: { children: React.ReactNode }) {
+  // Always start with 'en' to avoid hydration mismatch
+  // Then sync with localStorage after mount
+  const [locale, setLocaleState] = useState<Locale>('en');
+  const [mounted, setMounted] = useState(false);
+
+  // Get messages based on current locale
+  const messages = locale === 'zh' ? zhMessages : enMessages;
+
+  // Load locale from localStorage after component mounts (client-side only)
+  useEffect(() => {
+    setMounted(true);
+    if (typeof window !== 'undefined') {
+      const stored = localStorage.getItem(LOCALE_STORAGE_KEY);
+      if (stored === 'zh') {
+        setLocaleState('zh');
+      }
+    }
+  }, []);
+
+  // Update locale and persist to localStorage
+  const setLocale = (newLocale: Locale) => {
+    setLocaleState(newLocale);
+    if (typeof window !== 'undefined') {
+      localStorage.setItem(LOCALE_STORAGE_KEY, newLocale);
+      // Update HTML lang attribute
+      document.documentElement.lang = newLocale;
+    }
+  };
+
+  // Set HTML lang attribute when locale changes
+  useEffect(() => {
+    if (typeof window !== 'undefined' && mounted) {
+      document.documentElement.lang = locale;
+    }
+  }, [locale, mounted]);
+
+  return (
+    <LocaleContext.Provider value={{ locale, messages, setLocale }}>
+      {children}
+    </LocaleContext.Provider>
+  );
+}
+
+export function useLocaleContext() {
+  const context = useContext(LocaleContext);
+  if (context === undefined) {
+    throw new Error('useLocaleContext must be used within a LocaleProvider');
+  }
+  return context;
+}
+