fix: finality_delay = block_confs - 1

[Kodylow]

fixes: https://github.com/fedimint/ui/issues/379

[justinmoon]

What happens if a user enters 0 into the "block confirmations" field?

[Kodylow]

there's a min check in the form element:

<NumberFormControl
                labelText={t('set-config.block-confirmations')}
                helperText={t('set-config.block-confirmations-help')}
                warningText={t('set-config.block-confirmations-warning')}
                recommendedMin={6}
                min={1}
                max={200}
                value={blockConfirmations}
                onChange={(value) => {
                  setBlockConfirmations(value);
                }}
              />

[Kodylow]