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/i18n/request.ts

@@ -0,0 +1,22 @@
+import {getRequestConfig} from 'next-intl/server';
+import {routing} from './routing';
+
+/**
+ * Configuration for internationalization request handling
+ * This function is called for each request to determine the locale and load translations
+ */
+export default getRequestConfig(async ({requestLocale}) => {
+  // This typically corresponds to the `[locale]` segment
+  let locale = await requestLocale;
+
+  // Ensure that the incoming `locale` is valid
+  if (!locale || !routing.locales.includes(locale as any)) {
+    locale = routing.defaultLocale;
+  }
+
+  return {
+    locale,
+    messages: (await import(`../messages/${locale}.json`)).default
+  };
+});
+

surfsense_web/i18n/routing.ts

@@ -0,0 +1,24 @@
+import {defineRouting} from 'next-intl/routing';
+import {createNavigation} from 'next-intl/navigation';
+
+/**
+ * Internationalization routing configuration
+ * Defines supported locales and routing behavior for the application
+ */
+export const routing = defineRouting({
+  // A list of all locales that are supported
+  locales: ['en', 'zh'],
+
+  // Used when no locale matches
+  defaultLocale: 'en',
+
+  // The `localePrefix` setting controls whether the locale is included in the pathname
+  // 'as-needed': Only add locale prefix when not using the default locale
+  localePrefix: 'as-needed'
+});
+
+// Lightweight wrappers around Next.js' navigation APIs
+// that will consider the routing configuration
+export const {Link, redirect, usePathname, useRouter, getPathname} =
+  createNavigation(routing);
+