[RAPID-micro] 메신저 입력창 상단 가로선 제거

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

frontend/components/messenger/MessageInput.tsx

@@ -191,7 +191,7 @@ function MessageInput({ roomId, onTypingStart, onTypingStop }: MessageInputProps
 
   return (
     <div
-      className="border-t relative shrink-0"
+      className="relative shrink-0"
       onDragOver={(e) => e.preventDefault()}
       onDrop={handleDrop}
     >