feat: consumer-facing invoice checkout page (#71)

[0xdevcollins]

Summary

• Adds public GET /v1/invoices/:id/checkout endpoint returning merchant branding, line items, and invoice totals
• Adds public POST /v1/invoices/:id/pay that creates a 1:1 fiat quote + payment session and returns paymentId
• Patches getCheckoutPayment to expose paymentMethods from payment metadata so existing PaymentMethodSelector renders correctly
• Adds useInvoiceCheckout and useInitiateInvoicePayment hooks in the checkout app
• Builds full InvoiceCheckoutClient component handling all invoice states:
  • Loading — skeleton UI
  • Draft — not yet sent notice
  • Paid — confirmation with paid date
  • Cancelled — cancelled notice
  • Overdue — red warning + Pay button still available
  • Payable (SENT / VIEWED / PARTIALLY_PAID) — line items table, totals, Pay Now button with merchant brand color
• Adds /invoice/[invoiceId] route in apps/checkout

Test plan

• Open a SENT invoice checkout URL — verify merchant branding, line items, and Pay Now button render correctly
• Click Pay Now — verify redirect to /[paymentId] checkout flow
• Open a PAID invoice — verify paid confirmation state shown, Pay Now hidden
• Open a CANCELLED invoice — verify cancelled state shown
• Open a DRAFT invoice — verify "not yet available" message shown
• Open an OVERDUE invoice — verify red warning and Pay button still visible
• Open a PARTIALLY_PAID invoice — verify balance due shown and Pay button available
• Verify brand color applied to Pay Now button when merchant has brandColor set
• Verify mobile layout renders without iframe issues

Closes #71

🤖 Generated with Claude Code