admin: allow deleting users from the site (parity with project Danger Zone)

[themightychris]

Context

Reported by Travis in #0-leadership-team (June 2026), alongside the home-page CTA fix (PR #128):

while logged into my admin account, I can edit users but cannot delete them from the site which would be handy.
I do have "The Danger Zone" with delete for projects though.

Current state

• Admins can edit users but there's no delete affordance for them.
• Projects already have a "Danger Zone" with delete — so there's an established pattern + UX to mirror.

Ask

Add an admin-only user-delete affordance with parity to the project Danger Zone. Worth confirming against the spec what "delete a user" should mean in the gitsheets model (hard delete vs. soft-delete/tombstone, handling of authored content, slug retention for legacy-id redirects) before implementing — likely needs a spec note in specs/screens/account.md / the relevant person-admin spec and specs/behaviors/authorization.md.

Related: soft-delete handling is touched on in #113.