feat: improve AI Chat UX, add notebook summary, and fix shared/reminders routing

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

@@ -1,6 +1,7 @@
 /**
  * Detect user's preferred language.
  * Priority:
+ * 0. user-language cookie (explicit user choice, synced with localStorage)
  * 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'
@@ -8,7 +9,7 @@
 
 import { auth } from '@/auth'
 import { prisma } from '@/lib/prisma'
-import { unstable_cache } from 'next/cache'
+import { unstable_cache, cookies } from 'next/cache'
 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'])
@@ -79,6 +80,16 @@ const getCachedUserLanguage = unstable_cache(
  *                               Should be passed from server components that have access to headers().
  */
 export async function detectUserLanguage(browserLanguageHint?: SupportedLanguage | null): Promise<SupportedLanguage> {
+  // 0. Cookie has highest priority — set by the client when user picks a language.
+  //    This guarantees server and client agree on the language (no hydration mismatch).
+  try {
+    const cookieStore = await cookies()
+    const cookieLang = cookieStore.get('user-language')?.value
+    if (cookieLang && SUPPORTED_LANGUAGES.has(cookieLang)) {
+      return cookieLang as SupportedLanguage
+    }
+  } catch {}
+
   const session = await auth()
 
   if (!session?.user?.id) {