docs: enable contextIsolation in fiddles (#39613)

docs/fiddles/native-ui/dialogs/error-dialog/main.js

@@ -1,5 +1,6 @@
 // Modules to control application life and create native browser window
 const { app, BrowserWindow, ipcMain, dialog, shell } = require('electron/main')
+const path = require('node:path')
 
 // Keep a global reference of the window object, if you don't, the window will
 // be closed automatically when the JavaScript object is garbage collected.
@@ -11,8 +12,7 @@ function createWindow () {
     width: 800,
     height: 600,
     webPreferences: {
-      contextIsolation: false,
-      nodeIntegration: true
+      preload: path.join(__dirname, 'preload.js')
     }
   })