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/content/docs/meta.json

@@ -2,5 +2,5 @@
 	"title": "Setup",
 	"description": "The setup guide for Surfsense",
 	"root": true,
-	"pages": ["---Setup---", "index", "installation", "docker-installation", "manual-installation"]
+	"pages": ["index", "installation", "docker-installation", "manual-installation"]
 }