Default refund amount to blank or zero

Change the refund amount field so it does not default to the full refundable amount when starting a refund.

Transcript Context

Catherine said the current refund flow defaults to the full fee, which has led staff to accidentally issue full refunds when they only meant to issue partial refunds. She said this has already happened multiple times and that asking customers to pay again after an accidental full refund has not gone well.

The requested behavior was simple: make the amount empty or zero by default so staff must enter the intended refund amount intentionally before submitting.