Send confirmation to pending email with reconfirmation-specific copy

[maebeale]

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

• When an admin changes a user's email, the confirmation email was sent to the old email instead of the new (pending) one
• The confirmation email used welcome/invite copy ("Set your password") even for existing users confirming an email change — confusing for the recipient

How did you approach the change?

• Override send_confirmation_instructions on User to always route to unconfirmed_email when present, rather than relying on Devise's pending_reconfirmation? check
• Branch the DeviseMailer confirmation_instructions subject line: welcome vs reconfirmation
• Branch the HTML and text email templates with appropriate copy for each scenario:
  • Welcome invite (new users): unchanged — "Welcome to the AWBW Portal!", "Set your password"
  • Reconfirmation (email change): "Confirm your new email", shows the new email address, safe-to-ignore note
• Add mailer specs covering both branches (subject, body content, new email shown)

UI Testing Checklist

• Create a new user → they receive the welcome invite email with "Set your password"
• Change an existing confirmed user's email → confirmation email goes to the new email with "Confirm your new email" copy
• Use "Resend confirmation email" on confirm_email_manual page for a pending email change → email goes to the pending email
• Use the interstitial "Continue" button after email change → email goes to the pending email

Anything else to add?

• The ProcessEmailManualConfirm and ProcessEmailChange services both call send_confirmation_instructions, so the User model override fixes both flows in one place

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