Logged-in users can change their email address safely

[Odzen]

Replaces old issue #886.

Problem

Users can edit some profile contact data, but changing the account email must update both Firebase Auth and Mongo-backed records. It also needs notification and verification behavior so users do not get locked out or create split identity records.

Required behavior

• User can see current account email and submit a new email from profile/account settings.
• Require reauthentication before changing email.
• Update Firebase Auth email.
• Update email across Mongo profile/application/user records.
• Send notification to the old email and confirmation/verification to the new email.
• Keep role-specific login behavior intact.

Acceptance criteria

• New email is validated and rejected if already in use.
• Firebase and Mongo records stay consistent after a successful change.
• Old and new email addresses receive the appropriate notifications.
• User must verify the new email if required by Firebase policy.
• Existing sessions are handled according to a documented policy.
• Multi-language email subjects/body data are supported where existing templates support it.