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/assets/css/design.css

@@ -250,6 +250,17 @@ textarea:hover{
     border: #646E76 solid 1px;
     color: #ffffff;
     font-size: 16px;
+    min-width: 160px;
+    max-width: 100%;
+    height: 32px;
+    line-height: 32px;
+    padding: 0 28px 0 8px;
+    box-sizing: border-box;
+    vertical-align: middle;
+}
+
+.component-line .ui-select{
+    width: calc(100% - 72px);
 }