feat: add invoice cancellation UI

[newmattock]

Fixes #14

Summary

• add a reusable CancelInvoiceButton with the required irreversible-action confirmation modal
• show Cancel Invoice only to the invoice submitter while the invoice is Pending
• wire cancellation into the invoice detail page and freelancer invoice list, with optimistic local status updates to Cancelled
• submit cancel_invoice() through the existing signed transaction flow and surface success/error toasts
• add focused component coverage plus invoice-detail integration coverage
• carry narrow current-main build fixes for governance status typing and related TypeScript compatibility issues

Validation

• npm test -- src/components/__tests__/CancelInvoiceButton.test.tsx app/pay/[id]/__tests__/PayInvoice.test.tsx --reporter verbose
• npm run lint -- src/components/CancelInvoiceButton.tsx src/components/__tests__/CancelInvoiceButton.test.tsx app/pay/[id]/page.tsx app/pay/[id]/__tests__/PayInvoice.test.tsx app/freelancer/page.tsx app/governance/page.tsx src/utils/federation.ts src/utils/evidence.ts src/components/TokenSelector.tsx
• npm run build
• git diff --check
• local smoke: curl -I http://localhost:3028/freelancer returned HTTP/1.1 200 OK
• local smoke: curl -I http://localhost:3028/pay/1 returned HTTP/1.1 200 OK