feat: Update database setup and download handling

- Set a fixed database password and ensure old data is overwritten during setup.
- Enhance the download functionality with improved error handling for missing files, providing user-friendly HTML responses.
- Add instructions for placing client builds in the downloads directory in the admin panel.

admin-panel/src/pages/Downloads.tsx

@@ -152,6 +152,9 @@ export default function Downloads() {
             {building ? '构建中...' : '构建选中平台'}
           </button>
         </div>
+        <p className="text-sm text-gray-500 mb-4">
+          当前需在本地或 CI 中构建客户端，将产物（如 FunMC-版本号-windows-x64.exe）放入服务器 <code className="bg-gray-100 px-1 rounded">downloads</code> 目录（由环境变量 DOWNLOADS_DIR 指定，默认 /opt/funmc/downloads）后，下载页与下表才会提供下载。
+        </p>
 
         <div className="grid grid-cols-2 md:grid-cols-4 gap-4">
           {Object.entries(platformInfo).map(([key, info]) => (