3PID/E-mail Invite flow could be more user-friendly for logged out users

### Your use case

#### What would you like to do?

We are sending e-mail invites to people and hoping for them to figure out how to join our private rooms.

The flow for accepting the e-mail invites is not particularly user-friendly for users that aren't logged in to our Element Web instance, however.

1) Upon clicking the link in the e-mail, we see this screen:

Particular flaws:

- no acknowledgement that we have an e-mail invite. It's not reassuring that the invite process is working.
- too many buttons that we could get lost in

Some of the overwhelm might be because guest access is enabled on this server.

Enhancements:

- message acknowledging the state: 'You've been invited to join a conversation. Sign up or sign in to accept your invitation.'
- hide the other UI elements and focus on the Sign up / Sign in

2) If the user now presses 'Sign in', once they have logged in, the invite context is lost and they are just presented with the normal homepage

Enhancements:

- Memorise the state before login and return to the e-mail invite automatically

3) User has to open the link again

This is definitely not terrible, but it looks like an error at first glance — bold heading 'This invite .... not associated with your account'

The wording underneath 'Link this email with your account in settings'... sounds like I need to go and do that right now.

The 'Join the discussion' button seems fine but I wonder if the wording before that point will have scared some users off.

Enhancements:

- Soften the error-sounding heading — focus on the fact that you're about to accept e-mail invite first and foremost
- Make it clearer that 'Link this email in settings' is a **hint** and **for future invites**, not a strict requirement for right now

I think the text currently displayed here could be put in a 'Tip!' box or something of the sort, ideally not taking up the main focus of the screen which should first and foremost about clicking the accept button.

#### Why would you like to do it?

We are having reports of users not managing to complete the flow.

#### How would you like to achieve it?

-

### Have you considered any alternatives?

-

### Additional context

_No response_