HOLD UNTIL REQUESTED: Let admins register attendees without sending them a confirmation email

[maebeale]

Closes [link an issue or remove this line]

What is the goal of this PR and why is this important?

• Staff frequently register people on their behalf, where the automatic "you're registered" email to the attendee is unwanted noise (and sometimes goes to someone who never asked to be emailed).
• This adds an admin-only "on behalf" mode to the public registration form that captures the full registration but suppresses the registrant-facing confirmation — while still sending the staff FYI notification.

How did you approach the change?

• PublicRegistration gained a send_confirmation: flag; the controller only honors on_behalf for users who can manage the event, finds/creates the registrant from the form (not the admin), and keeps the admin FYI.
• Because the on-behalf attendee isn't logged in, the form now renders the full logged-out field set for admins and reveals the identity fields it normally hides for a logged-in user via a small on-behalf Stimulus controller, with "Register on behalf" entry points on the registrants roster and the event Form actions menu.

UI Testing Checklist

• As an admin, open an event's registrants page and click "Register on behalf" — the toggle is pre-checked and the name/email identity fields are visible.
• Submitting in on-behalf mode creates the registration but sends no confirmation email to the attendee (staff FYI still arrives).
• Unchecking "Registering on someone's behalf" re-hides the identity fields; re-checking reveals them.
• As a logged-out visitor, the on-behalf toggle is not present and registration still emails a confirmation.

Anything else to add?

• The existing admin "New registration" → confirm-page flow (opt-in email) is unchanged; this complements it for the full-form case. Covered by new request and service specs.

[maebeale]

🤖 From Claude: Gated on allowed_to?(:dashboard?, @event) so a logged-out visitor can never set on_behalf to suppress their own confirmation — the flag is ignored unless the requester can manage the event.

[maebeale]

🤖 From Claude: Admins always get the full logged-out field set because the person being registered isn't logged in. The diff against the logged-in set (@behalf_only_field_ids) is what the Stimulus controller reveals/hides as the toggle changes.

[maebeale]

@jmilljr24 this offers an alternative to as_visitor. i reuse this controller in subsequent pr to offer a similar option for the bulk payment form. lmk what you think!