feat: build invoice token conversion UI

[newmattock]

Fixes #70.

Summary

• Add the missing submitter-facing /i/[id] invoice detail route that existing dashboard links already target.
• Add ChangeInvoiceTokenModal with current-token display, allowlisted replacement-token selector, and the required denomination warning.
• Show Change Token only when the connected wallet is the invoice freelancer and the invoice is still Pending; non-submitters get an explanatory restriction message.
• Add buildConvertInvoiceTokenTransaction() for convert_invoice_token(invoice_id, new_token) and a small useTransaction helper for prepare/sign/submit state.
• Optimistically update the invoice token display after a successful conversion.

Validation

• npm test -- src/components/__tests__/ChangeInvoiceTokenModal.test.tsx 'app/i/[id]/__tests__/InvoiceDetailPage.test.tsx' --reporter verbose
• npx eslint 'app/i/[id]/page.tsx' 'app/i/[id]/__tests__/InvoiceDetailPage.test.tsx' src/components/ChangeInvoiceTokenModal.tsx src/components/__tests__/ChangeInvoiceTokenModal.test.tsx src/hooks/useTransaction.ts vitest.config.ts
• npm run build
• git diff --check
• Local route smoke: GET /i/1 returned HTTP 200 on the Next dev server.

Notes

• src/utils/soroban.ts still has pre-existing lint errors elsewhere in the file on current main, so I did not treat full-file lint there as a clean gate for this PR.