Prune or wire the vestigial Screen union

[sshdopey]

Context

src/types.ts defines a Screen union (including how, learn, connect) and an onNav(screen) pattern that the route-based navigation has largely superseded.

Scope

• Remove unused members or reconnect the union to real navigation so it reflects the routes.

Acceptance criteria

• The Screen type matches how navigation actually works.

Pointers

src/types.ts, src/screens/Landing.tsx, src/app/page.tsx.

[Elizabeth-dev270]

@Elizabeth-dev270 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Allow me fix this ASAP

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Elizabeth-dev270 to this issue.