- Invoice now includes service_fee_amount via PlatformFeeService (3% env-configurable)
- Review step and payment step both show fee breakdown (subtotal + service fee = total)
- Payment records the full total (including fee), not just the program price
- Success screen shows a proper printable invoice with items table, totals breakdown,
  payment status badge, and a print button that hides nav/sidebar
- Stores invoiceId for the computed invoiceForPrint property
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>