web: add the welcome onboarding flow
Four steps in one card — profile, follow & star, ssh keys, finish — with
the same content as the appview's /welcome templates. The step rail sits
at the top and the body slides left or right as you move between steps.
Data is mocked, nothing talks to an api yet.
Field and Textarea gained the variants the steps needed: sentence-case
labels, and a shorter box for the ssh key.
Signed-off-by: eti <eti@eti.tf>