Web 端本地缓存、首页与会员弹窗体验优化

新增 IndexedDB 与最近文件 L1 缓存、云保存/打开链路；首页支持下载与清空本地缓存；修复清空 storage 后语言初始化报错；优化 design2 背景加载与侧栏标签样式；完善会员激活/支付弹层样式；后端补充文件缩略图字段与 thumb 接口。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend-web/pages/index.web.html

@@ -140,6 +140,8 @@
 
   <script src="../js/common/auth-session.js"></script>
   <script src="../js/common/cloud-files.js"></script>
+  <script src="../js/common/soon-local-store.js"></script>
+  <script src="../js/common/soon-recent.js"></script>
   <script src="../vendor/layui/layui.js"></script>
   <script src="../vendor/js/qrcode.min.js"></script>