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/LanguageSwitcher.tsx

@@ -0,0 +1,56 @@
+'use client';
+
+import {useLocaleContext} from '@/contexts/LocaleContext';
+import {
+  Select,
+  SelectContent,
+  SelectItem,
+  SelectTrigger,
+  SelectValue,
+} from '@/components/ui/select';
+import {Globe} from 'lucide-react';
+
+/**
+ * Language switcher component
+ * Allows users to change the application language
+ * Persists preference in localStorage
+ */
+export function LanguageSwitcher() {
+  const {locale, setLocale} = useLocaleContext();
+
+  // Supported languages configuration
+  const languages = [
+    {code: 'en' as const, name: 'English', flag: '🇺🇸'},
+    {code: 'zh' as const, name: '简体中文', flag: '🇨🇳'},
+  ];
+
+  /**
+   * Handle language change
+   * Updates locale in context and localStorage
+   */
+  const handleLanguageChange = (newLocale: string) => {
+    setLocale(newLocale as 'en' | 'zh');
+  };
+
+  return (
+    <Select value={locale} onValueChange={handleLanguageChange}>
+      <SelectTrigger className="w-[160px]">
+        <Globe className="mr-2 h-4 w-4" />
+        <SelectValue>
+          {languages.find(lang => lang.code === locale)?.name || 'English'}
+        </SelectValue>
+      </SelectTrigger>
+      <SelectContent>
+        {languages.map((language) => (
+          <SelectItem key={language.code} value={language.code}>
+            <span className="flex items-center gap-2">
+              <span>{language.flag}</span>
+              <span>{language.name}</span>
+            </span>
+          </SelectItem>
+        ))}
+      </SelectContent>
+    </Select>
+  );
+}
+