Add interstitial page for admin event registration creation

[maebeale]

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

• When an admin creates an event registration, we no longer want to silently send emails
• Instead, show an interstitial confirmation page where the admin can choose which follow-up actions to take
• This gives admins control over whether to create a user account, send an invite, send a registration confirmation email, and/or send the admin FYI email

How did you approach the change?

• Removed automatic email sending from EventRegistrationsController#create
• Added confirm (GET) and process_confirm (POST) member routes on event_registrations
• After a successful admin create, redirect to the new confirm interstitial page instead of the final destination
• The interstitial shows a registration summary and contextual checkboxes:
  • Create user account — only shown when the Person has no User and has an email
  • Send system invite email — shown when no User (to be created) or existing unconfirmed User
  • Send registration confirmation email — shown when Person has an email
  • Send admin notification email — always shown
• Created EventRegistrationServices::ProcessConfirmation service to handle the four optional actions
• Non-admin users retain the existing direct redirect behavior (no interstitial)
• Added admin-only confirm? and process_confirm? policy rules

UI Testing Checklist

• As admin, create an event registration from the manage event page — verify redirect to interstitial
• Verify checkboxes appear/hide based on Person's User account status
• Submit with no checkboxes selected — verify "Registration created." flash and correct redirect
• Submit with confirmation email checked — verify email is sent
• Submit with create user + send invite checked — verify user created and invite sent
• Click "Skip" — verify redirect without any actions
• As regular user, create a registration — verify no interstitial (direct redirect)

Anything else to add?

• The interstitial page follows existing admin page styling (admin-only bg-blue-100 background, white card)
• The service follows the same patterns as UsersController for user creation and PublicRegistration for notification sending

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