fix: 관리자 다운로드 폼 에러 피드백 추가 및 nginx HTML 캐싱 방지

- 저장 실패 시 에러 메시지 표시
- nginx index.html Cache-Control: no-store 설정

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/admin/AdminCommon.css

@@ -67,6 +67,12 @@
   border-color: rgba(186, 205, 176, 0.5);
 }
 
+.admin-error {
+  font-size: 0.85rem;
+  color: rgba(229, 115, 115, 0.9);
+  margin: 0;
+}
+
 .admin-form-actions {
   display: flex;
   gap: 8px;

src/components/admin/DownloadAdmin.jsx

@@ -7,6 +7,7 @@ export default function DownloadAdmin() {
   const [form, setForm] = useState({ url: '', version: '', fileName: '', fileSize: '', fileHash: '' });
   const [loading, setLoading] = useState(false);
   const [saved, setSaved] = useState(false);
+  const [error, setError] = useState('');
 
   useEffect(() => {
     getDownloadInfo()
@@ -23,6 +24,7 @@ export default function DownloadAdmin() {
   const handleSubmit = async (e) => {
     e.preventDefault();
     setLoading(true);
+    setError('');
     try {
       await apiFetch('/api/download/info', {
         method: 'PUT',
@@ -30,6 +32,8 @@ export default function DownloadAdmin() {
       });
       setSaved(true);
       setTimeout(() => setSaved(false), 2000);
+    } catch (err) {
+      setError(err.message || '저장에 실패했습니다.');
     } finally {
       setLoading(false);
     }
@@ -56,6 +60,7 @@ export default function DownloadAdmin() {
         {field('파일명', 'fileName', 'A301_Launcher.zip')}
         {field('파일 크기', 'fileSize', '1.2 GB')}
         {field('A301.exe SHA256 해시', 'fileHash', 'sha256 해시값 (certutil -hashfile A301.exe SHA256)')}
+        {error && <p className="admin-error">{error}</p>}
         <div className="admin-form-actions">
           <button className="btn-admin-primary" type="submit" disabled={loading}>
             {saved ? '저장됨 ✓' : '저장'}