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>

backend-web/scripts/migrate-schema.php

@@ -0,0 +1,24 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * Incremental DB migration (reads config/local.php). Safe to run repeatedly.
+ * Usage: cd backend-web && php scripts/migrate-schema.php
+ */
+if (PHP_SAPI !== 'cli') {
+    fwrite(STDERR, "CLI only\n");
+    exit(1);
+}
+
+require dirname(__DIR__) . '/src/bootstrap.php';
+require __DIR__ . '/schema-migrate-lib.php';
+
+use Soon\Api\Core\Db;
+
+try {
+    $count = soon_schema_migrate(Db::pdo());
+    fwrite(STDOUT, 'migrate-schema: done (' . $count . " ALTER/CREATE executed)\n");
+} catch (Throwable $e) {
+    fwrite(STDERR, 'migrate-schema: ' . $e->getMessage() . PHP_EOL);
+    exit(1);
+}