Fix Http auth redirect & provide nice looking UI feedback (fixes #84)

[opatry]

Description

• redirect was done to minimize the time the auth code is visible to the user in the URL but the callbackFlow result was immediately sent not waiting for redirect to finish. Postponing the result once the redirect is done allows providing proper user feedback from browser instead of shutdown server state.
• was also the opportunity to provide nice looking user feedback with proper HTML & assets
• on macOS, the error & success automatically redirects to the app

Checklist

• I have read the CONTRIBUTING guide
• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing