Improve FedCM Fallback for Unsupported Browsers

[thhck]

Currently, if a user attempts to use our IDP’s FedCM feature on an unsupported browser (or without the necessary flags enabled ), they encounter errors. We want to handle these cases more gracefully by detecting unsupported environments and/or hiding FedCM options entirely . This way, users do not see confusing error messages that might lead them to think the system is broken.

I'm thinking to hide the "Register IdP to FedCM" button from the login page. If the browser have the IdentityProvider API available, we just trigger the register function ( withtout asking the user to click the button ) , otherwise we dont.

related: #66

[michielbdejong]

Or:

• if the user is not using Chrome, show 'switch to Chrome if you want to benefit from FedCM'
• if the user is using Chrome but doesn't have the flags enabled, show 'enable these flags if you want to benefit from FedCM'
• if the user is using Chrome with the flags enabled, show the button as it looks now.