fix: 允许删除内置块，仅保护兼容块

- 删除按钮 disabled 条件从 non-custom 改为 legacyPrompt
- 后端守卫同步更新，内置块可删除，兼容块禁止删除

panel.js

@@ -2206,7 +2206,7 @@ function _renderTaskBlockListItem(block, index, state) {
           data-task-action="delete-block"
           data-block-id="${_escHtml(block.id)}"
           type="button"
-          ${block.type === "custom" ? "" : "disabled"}
+          ${block.type === "legacyPrompt" ? "disabled" : ""}
         >
           删除
         </button>
@@ -2661,8 +2661,8 @@ function _deleteTaskBlock(blockId) {
     const index = blocks.findIndex((item) => item.id === blockId);
     if (index < 0) return null;
     const block = blocks[index];
-    if (block.type !== "custom") {
-      toastr.info("只有自定义块可以删除", "ST-BME");
+    if (block.type === "legacyPrompt") {
+      toastr.info("兼容块不可删除", "ST-BME");
       return null;
     }
     blocks.splice(index, 1);