fix: 3차 리뷰 LOW — useCallback, 중복 제거, 접근성 강화

- DownloadSection loadInfo useCallback 래핑
- DownloadAdmin reload 중복 → 단일 load 함수
- LoginPage/RegisterPage 에러 메시지 role="alert"

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

src/components/DownloadSection.jsx

@@ -1,4 +1,4 @@
-import { useState, useEffect } from 'react';
+import { useState, useEffect, useCallback } from 'react';
 import { useNavigate } from 'react-router-dom';
 import { useAuth } from '../context/useAuth';
 import { getDownloadInfo } from '../api/download';
@@ -14,13 +14,13 @@ export default function DownloadSection() {
   const { user } = useAuth();
   const navigate = useNavigate();
 
-  const loadInfo = () => {
+  const loadInfo = useCallback(() => {
     setReady(false);
     setLoadError(false);
     getDownloadInfo()
       .then((data) => { setInfo(data); setReady(true); })
       .catch(() => { setLoadError(true); setReady(true); });
-  };
+  }, []);
 
   useEffect(() => { loadInfo(); }, []);