Create a Google Sheet. Rename the first sheet tab to Tasks.
2
Open Extensions → Apps Script. Paste the script from
here → (click to view script),
then click Deploy → New deployment → Web App.
Set "Who has access" to Anyone.
3
Copy the Web App URL and paste it below.
Your data stays in your own Google Sheet. This app stores only your Web App URL in localStorage.
Apps Script Code
Copy all of this code and paste it into your Apps Script editor. Then deploy as a Web App.