remove plugin button from web UI

frontend/src/App.jsx

@@ -3130,9 +3130,11 @@ function Flow() {
             <button className="btn" onClick={copySnapshot} title="Copy workflow screenshot to clipboard">
               ⎘ Snapshot
             </button>
+            {window.pywebview && (
               <button className="btn" onClick={uploadPlugin} title="Upload a plugin (.py)">
                 ⊕ Plugin
               </button>
+            )}
           </div>
 
           <div className={`status-bar ${status.level}`}>{status.text}</div>