feat: build batch invoice submission UI

[newmattock]

Fixes #25

Summary

• add a standalone /invoices/batch route with CSV upload/paste and dynamic multi-row input modes
• parse and preview payer, amount, token, discount_rate, and due_date rows with inline validation and a 50-row cap
• show summary totals by token, valid-row counts, and estimated transaction fees
• call submit_invoices_batch atomically from the connected freelancer wallet and show per-row success/failure results
• include focused CSV parsing, validation, summary, and submission tests
• include current-main governance/federation build fixes

Validation

• npm test -- src/utils/tests/batchInvoiceSubmission.test.ts src/components/tests/BatchInvoiceSubmissionPage.test.tsx --reporter verbose
• npm run lint -- src/utils/batchInvoiceSubmission.ts src/components/BatchInvoiceSubmissionPage.tsx app/invoices/batch/page.tsx src/utils/tests/batchInvoiceSubmission.test.ts src/components/tests/BatchInvoiceSubmissionPage.test.tsx src/utils/soroban.ts app/governance/page.tsx src/utils/federation.ts
• npm run build
• git diff --check
• local HTTP smoke: HEAD /invoices/batch returned 200 on port 3023
• in-app browser smoke found the batch route title, CSV/form controls, preview table, summary, and submit action