fix: isolate detect-user-language from client bundle

Remove detectUserLanguage re-export from i18n barrel index.ts — it depends
on next/headers (server-only) and was being pulled into client component bundles
via sidebar.tsx → lib/i18n/index.ts, causing build failure.
Add 'server-only' guard to detect-user-language.ts to make the constraint explicit.

memento-note/lib/i18n/detect-user-language.ts

@@ -5,11 +5,15 @@
  * 1. Most common language among user's notes (DB GROUP BY)
  * 2. Browser language hint (passed from server component via Accept-Language)
  * 3. Default: 'en'
+ *
+ * SERVER ONLY — uses next/headers and Prisma. Never import from client components.
  */
+import 'server-only'
 
 import { auth } from '@/auth'
 import { prisma } from '@/lib/prisma'
-import { unstable_cache, cookies } from 'next/cache'
+import { unstable_cache } from 'next/cache'
+import { cookies } from 'next/headers'
 import { SupportedLanguage } from './load-translations'
 
 const SUPPORTED_LANGUAGES = new Set(['en', 'fr', 'es', 'de', 'fa', 'it', 'pt', 'ru', 'zh', 'ja', 'ko', 'ar', 'hi', 'nl', 'pl'])