Add default family selection for invite-only onboarding mode (#1174)

* Add default family selection for invite-only onboarding mode

When onboarding is set to invite-only, admins can now choose a default
family that new users without an invitation are automatically placed into
as members, instead of creating a new family for each signup.

https://claude.ai/code/session_01U9KgikKjV6xbyBZ5wMYsYx

* Restrict invite codes and onboarding settings to super_admin only

The Invite Codes section on /settings/hosting was visible to any
authenticated user via the show action, leaking all family names/IDs
through the default-family dropdown. This tightens access:

- Hide the entire Invite Codes section in the view behind super_admin?
- Add before_action :ensure_super_admin to InviteCodesController for
  all actions (index, create, destroy), replacing the inline admin? check
- Add ensure_super_admin_for_onboarding filter on hostings#update that
  blocks non-super_admin users from changing onboarding_state or
  invite_only_default_family_id

https://claude.ai/code/session_01U9KgikKjV6xbyBZ5wMYsYx

* Fix tests for super_admin-only invite codes and onboarding settings

- Hostings controller test: sign in as sure_support_staff (super_admin)
  for the onboarding_state update test, since ensure_super_admin_for_onboarding
  now requires super_admin role
- Invite codes tests: use super_admin fixture for the success case and
  verify that a regular admin gets redirected instead of raising StandardError

https://claude.ai/code/session_01U9KgikKjV6xbyBZ5wMYsYx

* Fix system test to use super_admin for self-hosting settings

The invite codes section is now only visible to super_admin users,
so the system test needs to sign in as sure_support_staff to find
the onboarding_state select element.

https://claude.ai/code/session_01U9KgikKjV6xbyBZ5wMYsYx

* Skip invite code requirement when a default family is configured

When onboarding is invite-only but a default family is set, the
claim_invite_code before_action was blocking registration before
the create action could assign the user to the default family.
Now invite_code_required? returns false when
invite_only_default_family_id is present, allowing codeless
signups to land in the configured default family.

https://claude.ai/code/session_01U9KgikKjV6xbyBZ5wMYsYx

---------

Co-authored-by: Claude <noreply@anthropic.com>

app/controllers/registrations_controller.rb

@@ -18,6 +18,11 @@ class RegistrationsController < ApplicationController
       @user.family = @invitation.family
       @user.role = @invitation.role
       @user.email = @invitation.email
+    elsif (default_family_id = Setting.invite_only_default_family_id).present? &&
+          Setting.onboarding_state == "invite_only" &&
+          (default_family = Family.find_by(id: default_family_id))
+      @user.family = default_family
+      @user.role = :member
     else
       family = Family.new
       @user.family = family