fix(web): 本地优先保存与 design 页体验修复

- 保存先写本地缓存，登录后可选云端同步；退出登录不再因文件操作跳转登录

- 修复 Layui 遮罩残留、design2 保存后线条、toast 被 finally 提前关闭

- 保存过程恢复 loading spinner；成功提示 2.5 秒

- 云端 payload 瘦身与体积超限提示；后端 schema 迁移与 FileService 容错

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

frontend-web/pages/index.web.html

@@ -139,6 +139,7 @@
   <script src="../js/common/auth-redirect.js"></script>
 
   <script src="../js/common/auth-session.js"></script>
+  <script src="../js/common/soon-toast.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>