onboarding: avoid asking for email address before waitlisting user

[jamesacklin]

Bifurcates the first stage of the signup flow based on hosting availability. Fixes TLON-2656

We're coming from this world:

Enter email address -> Check inventory -> (N) Join waitlist
                                       -> (Y) Enter password

...which seems like a bait-and-switch for their email address.

Instead:

Check inventory -> (N) Join waitlist -> Enter email
                -> (Y) Enter email   -> Enter password

Here are some videos of the change in action.

No inventory (waitlist):

Screen.Recording.2024-09-03.at.6.11.07.PM.mov

Normal signup:

Screen.Recording.2024-09-03.at.6.16.28.PM.mov

[jamesacklin]

Adding @e-shee for copy review. :)

[e-shee]

LGTM

[linear]

TLON-2656 Avoid asking user for email address before waitlisting them

[dnbrwstr]

Cool, code lgtm.