fix: prevent deletion of library procedures

- Skip delete button creation for readonly blocks (data-readonly)
- Add WCProcedure.delete() override that guards against readonly

wcblock.js

@@ -46,7 +46,7 @@ class WCBlock extends HTMLElement {
     });
 
     const isAlgoBlock = !!this.closest("#draw-area");
-    if (!(this instanceof WCStart) && !(this instanceof WCEnd)) {
+    if (!(this instanceof WCStart) && !(this instanceof WCEnd) && !this.dataset.readonly) {
       const btn = document.createElement("button");
       btn.className = "delete-btn";
       btn.textContent = "\u00D7";