Document how to check user authentication offline

[vlukashov]

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As a developer I want to find step-by-step guide with working code sample on how to make my login view work in offline environments so that I can build a login-protected app and make it work offline with the @PWA annotation.

DoD:

• The typescript/adding-custom-login-form-with-spring-security page describes how to implement login that does not require a constant backend connection
• In the PWA section of the docs there is a link to that material dubbed "Offline authentication checks in PWAs" page under the @PWA section of the docs which documents the approach to offline login taken in the Vaadin CRM for TypeScript:
  • how to check if the user is logged in or not when the app is offline
  • how to log the user out offline
  • the security aspect of the offline isUserLoggedIn implementation (based on https://discord.com/channels/732335336448852018/774366844684468284/779255233355579392)