feat: add invoice sharing links

[newmattock]

Fixes #23

Summary

• Add a canonical public /invoices/[id] route that reuses the existing invoice settlement/detail page.
• Add a submitter-only ShareInvoiceButton with clipboard copy for the canonical invoice URL and a prefilled email share link.
• Show the required Link copied! confirmation after successful copy.
• Add focused coverage for URL/mailto generation, clipboard copy, and submitter/non-submitter visibility.
• Carry narrow current-main build fixes needed for validation.

Validation

• npm test -- src/utils/__tests__/invoiceSharing.test.ts src/components/__tests__/ShareInvoiceButton.test.tsx 'app/pay/[id]/__tests__/PayInvoice.test.tsx' --pool forks --maxWorkers 1 --reporter verbose
• npm run lint -- src/utils/invoiceSharing.ts src/utils/__tests__/invoiceSharing.test.ts src/components/ShareInvoiceButton.tsx src/components/__tests__/ShareInvoiceButton.test.tsx 'app/pay/[id]/page.tsx' 'app/pay/[id]/__tests__/PayInvoice.test.tsx' 'app/invoices/[id]/page.tsx' app/governance/page.tsx src/components/TokenSelector.tsx src/utils/federation.ts
• git diff --check
• npm run build
• curl -I http://127.0.0.1:3036/invoices/1
• curl -I http://127.0.0.1:3036/pay/1