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/next.config.ts

@@ -1,5 +1,9 @@
 import { createMDX } from "fumadocs-mdx/next";
 import type { NextConfig } from "next";
+import createNextIntlPlugin from 'next-intl/plugin';
+
+// Create the next-intl plugin
+const withNextIntl = createNextIntlPlugin('./i18n/request.ts');
 
 const nextConfig: NextConfig = {
 	output: "standalone",
@@ -19,7 +23,7 @@ const nextConfig: NextConfig = {
 	},
 };
 
-// Wrap the config with createMDX
+// Wrap the config with MDX and next-intl plugins
 const withMDX = createMDX({});
 
-export default withMDX(nextConfig);
+export default withNextIntl(withMDX(nextConfig));