Is your feature request related to a problem? Please describe.

Using the Auth Emulator page for GitHub (may be the same for other Identity Providers):
http://localhost:4280/.auth/login/github

If the user enters invalid characters for Username, an attempt to submit the form fails "silently" in the UI.

Example value for Username:

یونیکد

The console gives additional information:

The string to be encoded contains characters outside of the Latin1 range.

Uncaught DOMException: Failed to execute 'btoa' on 'Window': The string to be encoded contains characters outside of the Latin1 range. at saveCookie (http://localhost:4280/.auth/login/github:168:54) at HTMLFormElement.<anonymous> (http://localhost:4280/.auth/login/github:316:11) at HTMLFormElement.dispatch (https://ajax.aspnetcdn.com/ajax/jquery/jquery-3.2.1.min.js:3:10316) at HTMLFormElement.q.handle (https://ajax.aspnetcdn.com/ajax/jquery/jquery-3.2.1.min.js:3:8343)

Describe the solution you'd like
If it is required that Username characters be in the Latin1 range, add a validation message near the Username field in the UI.
If the full range of Unicode characters is allowed, adjust logic, to allow such characters.

Describe alternatives you've considered
Open the DevTools Console while running, to view run-time exceptions

Additional context
Exception is thrown in this source code file:
https://github.com/Azure/static-web-apps-cli/blob/main/src/public/auth.html

      function saveCookie(formElement) {
        const data = localStorage[hashStorageKey(formElement)];
        document.cookie = `StaticWebAppsAuthCookie=${btoa(data)}; path=/`;
      }