dashboard: Worker Cron Job Scheduler Fails to Save

### What versions & operating system are you using?

Safari 26.5 and Chrome 148.0.7778.179

### Please provide a link to a minimal reproduction

https://plain-fire-41bd.bach-project-services.workers.dev

### Describe the Bug

When I am using the web to attempt to add a cronjob trigger, it fails to save. To reproduce the issue, I go to settings -> Add next to Trigger Events -> Cron triggers -> Add (the default doesn't work). When clicking the final Add button, nothing happens to the interface.

I also attempted to change the frequency to every 1 minute, every 1 hour, every 3 hours, and every 5 hours, none of which would save.

When I go to my browser console, I can see that there were 2 400 responses returned after clicking the save button. I then went to the network tab and see that the browser is sending a blank string as the cron schedule which fails on the validate and describe endpoints and therefore does not actually save.

Workaround: In the sidebar panel where you choose your frequency, choose the tab for "Cron expression". Here, you can write your own cron schedule the way you would with a cron job on any other server/computer.

The issue: the web interface is not interpreting the dropdown and text field inputs correctly and therefore sending a blank string to be validated which fails. Another related issue is that when this fails, there should be some form of an error message that appears to the user allowing them to make changes but this does not happen either, leaving the user confused on what happened.

### Please provide any relevant error logs

_No response_

再現用 URL から始め、Safari または Chrome で Settings → Add（Trigger Events の横）→ Cron triggers → Add の順に進みます。ブラウザーの Network パネルとコンソールパネルを使用して、保存リクエストが空の cron スケジュールを送信し、400 レスポンスを返すことを確認します。完了条件は、ドロップダウンとテキストフィールドの選択によって有効なスケジュールが生成され、正常に保存され、保存に失敗した場合にエラーが表示されることです。

索引モデルが issue の本文から書いたものです。