feat: 관리자 페이지 추가 (공지사항, 다운로드, 유저 관리)

- /admin 라우트 추가 (admin 권한 전용)
- 공지사항 CRUD, 다운로드 정보 수정, 유저 권한/삭제 관리
- AuthContext에 role 추가 및 localStorage 저장
- 홈 헤더에 admin 링크 표시 (admin만 노출)

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

src/components/admin/DownloadAdmin.jsx

@@ -0,0 +1,65 @@
+import { useState, useEffect } from 'react';
+import { getDownloadInfo } from '../../api/download';
+import { apiFetch } from '../../api/client';
+import './AdminCommon.css';
+
+export default function DownloadAdmin() {
+  const [form, setForm] = useState({ url: '', version: '', fileName: '', fileSize: '' });
+  const [loading, setLoading] = useState(false);
+  const [saved, setSaved] = useState(false);
+
+  useEffect(() => {
+    getDownloadInfo()
+      .then((data) => setForm({
+        url: data.url,
+        version: data.version,
+        fileName: data.fileName,
+        fileSize: data.fileSize,
+      }))
+      .catch(() => {});
+  }, []);
+
+  const handleSubmit = async (e) => {
+    e.preventDefault();
+    setLoading(true);
+    try {
+      await apiFetch('/api/download/info', {
+        method: 'PUT',
+        body: JSON.stringify(form),
+      });
+      setSaved(true);
+      setTimeout(() => setSaved(false), 2000);
+    } finally {
+      setLoading(false);
+    }
+  };
+
+  const field = (label, key, placeholder) => (
+    <div className="admin-field">
+      <label className="admin-label">{label}</label>
+      <input
+        className="admin-input"
+        placeholder={placeholder}
+        value={form[key]}
+        onChange={(e) => setForm({ ...form, [key]: e.target.value })}
+      />
+    </div>
+  );
+
+  return (
+    <div className="admin-section">
+      <h2 className="admin-section-title">다운로드 정보 관리</h2>
+      <form className="admin-form" onSubmit={handleSubmit}>
+        {field('다운로드 URL', 'url', 'https://...')}
+        {field('버전', 'version', 'v1.0.0')}
+        {field('파일명', 'fileName', 'A301_Launcher.exe')}
+        {field('파일 크기', 'fileSize', '1.2 GB')}
+        <div className="admin-form-actions">
+          <button className="btn-admin-primary" type="submit" disabled={loading}>
+            {saved ? '저장됨 ✓' : '저장'}
+          </button>
+        </div>
+      </form>
+    </div>
+  );
+}