fix: pass language param from floating AI chat to API so it uses localized system prompt

The floating ai-chat.tsx was not sending the user's language to the
/api/chat endpoint, causing it to always use the English system prompt.
Now passes `language` from useLanguage() hook.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

memento-note/components/ai-chat.tsx

@@ -66,7 +66,8 @@ export function AIChat() {
           chatScope,
           notebookId: chatScope !== 'all' ? chatScope : undefined,
           webSearch: webSearch && webSearchAvailable,
-          conversationId
+          conversationId,
+          language,
         }
       }
     )