fix(super-admin): prefill confirmed_at in new user form (#13662)

On self-hosted instances without email configured, users created from
Super Admin can get stuck in an unconfirmed state. This PR implements
the default at the Super Admin frontend form layer, not in backend
creation logic.

What changed:
- Added a custom `ConfirmedAtField` for Super Admin user forms.
- Prefills `confirmed_at` with current time on the **New User** form
(`GET /super_admin/users/new`).
- Kept backend create behavior unchanged
(`resource_class.new(resource_params)`), so API/manual payloads still
behave normally.

Behavior:
- In Super Admin UI, `confirmed_at` is prefilled by default.
- If someone wants an unconfirmed user, they can clear the
`confirmed_at` field before saving.
- If `confirmed_at` is omitted from payload entirely, the created user
remains unconfirmed.

Scope note: external signup flows are intentionally unchanged in this PR
(`/api/v1/accounts`, `/api/v2/accounts`, and social/omniauth signup
behavior are not modified).

## Demo 





https://github.com/user-attachments/assets/436abbb0-d4cf-49a6-a1b8-4b6aa85aa09f

app/fields/confirmed_at_field.rb

@@ -0,0 +1,4 @@
+require 'administrate/field/base'
+
+class ConfirmedAtField < Administrate::Field::DateTime
+end