Allow email change

Notes:
- we don't know the current user email (one-way encrypted) and thus can't display it => there is a chance that users changes their email... for the same email again (they might have also forgotten which email they used initially). We should consider this ok, and could tell the user "actually you already used this email, that's fine".
- if an account already exists for the new email, this means we might need reconciliation, see #302. We could consider the most recent response as the canonical one.

This means both this feature and #302 are dependent on also creating an interface that shows user multiple response and letting them delete one or maybe "fuse" them (like keeping the most recent response for each field).

Start by reading issue #302, then map how email changes and duplicate accounts are handled in the TypeScript monorepo. The scope includes accepting reuse of an already-used email and providing an interface to review, delete, or fuse multiple responses; done requires agreement on the reconciliation behavior and its user-facing flow.

Written by the indexing model from the issue text.