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/UserAdmin.jsx

@@ -0,0 +1,49 @@
+import { useState, useEffect } from 'react';
+import { getUsers, updateUserRole, deleteUser } from '../../api/users';
+import { useAuth } from '../../context/AuthContext';
+import './AdminCommon.css';
+
+export default function UserAdmin() {
+  const [users, setUsers] = useState([]);
+  const { user: me } = useAuth();
+
+  const load = () => getUsers().then(setUsers);
+  useEffect(() => { load(); }, []);
+
+  const handleRoleToggle = async (u) => {
+    const newRole = u.role === 'admin' ? 'user' : 'admin';
+    if (!confirm(`${u.username}의 권한을 ${newRole}로 변경하시겠습니까?`)) return;
+    await updateUserRole(u.id, newRole);
+    load();
+  };
+
+  const handleDelete = async (u) => {
+    if (!confirm(`${u.username} 계정을 삭제하시겠습니까?`)) return;
+    await deleteUser(u.id);
+    load();
+  };
+
+  return (
+    <div className="admin-section">
+      <h2 className="admin-section-title">유저 관리</h2>
+      <ul className="admin-list">
+        {users.map((u) => (
+          <li key={u.id} className="admin-list-item">
+            <div className="admin-list-info">
+              <span className="admin-list-title">{u.username}</span>
+              <span className={`admin-role-badge ${u.role}`}>{u.role}</span>
+            </div>
+            {u.username !== me?.username && (
+              <div className="admin-list-actions">
+                <button className="btn-admin-edit" onClick={() => handleRoleToggle(u)}>
+                  {u.role === 'admin' ? '일반으로' : '관리자로'}
+                </button>
+                <button className="btn-admin-delete" onClick={() => handleDelete(u)}>삭제</button>
+              </div>
+            )}
+          </li>
+        ))}
+      </ul>
+    </div>
+  );
+}