feat: add due date reminder opt-in

[newmattock]

Fixes #24

Summary

• add an invoice-party-only reminder prompt on the invoice detail/settlement page
• wrap Browser Notification API permission and scheduling for 24-hour advance due-date reminders
• persist per-invoice reminder preferences in localStorage and hide the prompt after denied permission
• schedule the required Invoice #[ID] is due tomorrow browser notification
• add focused coverage for preference keys, delay calculation, notification firing, permission states, party gating, and pay-page wiring
• include narrow current-main build fixes for governance Vetoed status/deferred refresh and typed federation home-domain access

Validation

• npm test -- src/utils/tests/invoiceReminders.test.ts src/components/tests/RemindMeButton.test.tsx app/pay/[id]/tests/PayInvoice.test.tsx --reporter verbose
• npx eslint src/utils/invoiceReminders.ts src/utils/tests/invoiceReminders.test.ts src/components/RemindMeButton.tsx src/components/tests/RemindMeButton.test.tsx app/pay/[id]/page.tsx app/pay/[id]/tests/PayInvoice.test.tsx app/governance/page.tsx src/utils/federation.ts vitest.config.ts
• npm run build
• git diff --check
• local smoke: HEAD /pay/1 returned HTTP 200 on port 3021
• in-app browser smoke: /pay/1 exposed invoice page content