Duplicate connection errors now name the existing account

Connecting an account that is already linked to a user has always been rejected with 409 Conflict. That response now carries existing_institution_id, so you can point the user at the account they already have.

Direct API

POST /accounts and PUT /accounts/:id return it alongside the existing message:

{
  "message": "This connection is already linked to your account.",
  "existing_institution_id": "<id of the already-linked account>"
}

Connect widget

The DUPLICATE_CONNECTION error event now includes it at the top level of the postMessage payload:

{
  event: "ERROR",
  data: {
    type: "DUPLICATE_CONNECTION",
    existing_institution_id: "<id of the already-linked account>",
    error: { /* unchanged */ }
  }
}

Action required

None — this is purely additive and existing 409 handling keeps working. If you want to use it, the field lets you replace a generic error with something like "you already have this account connected", linked to the existing connection.