[Transfers] Add idempotency key to UI form

Prevent a double click on submit button from creating two disbursements/transfers. We'll eventually add it to the API.

Ticket created because of this: https://hackclub.slack.com/archives/CN523HLKW/p1770307026474539
However, we determined that the specific example (in the link) above of a duplicate disbursement was due to user error (requested 1 minute apart from each other), rather than a double click.

Nevertheless, we should add more protection

To implement this, please:
1. Add a nullable idempotency_key string column to all transfer models (AchTransfer, Check, etc.)
2. Ensure the idempotency_key column is unique
3. Hook up the transfer forms to pass in a hidden idempotency_key form field/param.
4. In Javascript, generate a UUID for the idempotency_key. Make this reuseable (e.g. via a partial & stimulus controller).
5. Have the controller raise a friendly error when idempotency_key is found to be not unique.

Locate the AchTransfer, Check, and other transfer models, their forms, and the relevant controller actions. Start by tracing how a transfer form is submitted and how transfer records are created, then inspect the existing JavaScript and Stimulus patterns for reusable form behavior. Done means every transfer form sends a generated UUID, duplicate keys are rejected with a friendly controller error, and the uniqueness constraint is covered by tests.

Written by the indexing model from the issue text.