feat(send): wire confirm step to /api/send and computeAllocation

[Wetshakat]

closed #456

Summary

feat(send): Wire confirm step to real /api/send endpoint and compute allocation

Description

This PR replaces the simulated transaction flow in the Send Money feature with a production-ready implementation. It wires the Review step directly to the /api/send endpoint, integrates the remittance split logic client-side, and introduces robust error handling, pending states, and i18n support.

Changes Included

🏗 Backend & Typings

• Strict API Typings: Added SendTransactionRequest, SendTransactionResponse, and SendTransactionResult to lib/types/api.ts to enforce a unified contract across the frontend and backend.
• Endpoint Validation: Tightened the /api/send stub (app/api/send/route.ts) to validate the request payload (checks for amount > 0 and missing recipients) before returning a typed placeholder payload.

🖥 Frontend & UX

• Removed Mocks: Removed the hardcoded "Maria Santos", fabricated hashes, and inline split math (amount * 0.5) from app/send/page.tsx.
• Real API Integration: The submission flow now uses apiClient.post, gracefully handling session expirations, 4xx/5xx API failures, and network timeouts.
• Dynamic Split Breakdown: We now use computeAllocation() (imported from lib/remittance/split.ts) to generate integer-safe allocation splits dynamically based on the sent amount.
• Pending States & Accessibility: Updated ReviewStep.tsx to accept an isPending prop. The Confirm button now disables during flight, shows a spinning loader, and uses aria-busy / dynamic aria-labels for screen readers.
• Receipt Component Fix: Updated TransactionSuccessReceipt to accept splits.spending rather than the mismatched dailySpending property.

🌍 Localization

• Externalized all user-facing strings (success/error toasts, confirm button text, loading states) to the send namespace in en.json and es.json.

🧪 Testing & Documentation

• Unit Tests: Added tests/unit/send-handler.test.ts (Vitest-compatible) which asserts:
  • Input validation boundaries (empty amount, missing recipient)
  • Happy path verification (proper split math, fallback recipient truncation)
  • Error path execution (4xx, 5xx, and generic network failures)
• README Update: Appended a new "Send Flow Integration" section to README_SPLIT_TRANSACTIONS.md outlining why the app calculates the split breakdown client-side.

Testing Instructions

1. Check out this branch locally: git checkout feat/connect-send-money-api-flow.
2. Start the dev server (npm run dev) and navigate to the Send flow.
3. Advance to the Review step and confirm a transaction. Observe the loading state and the real success receipt populated with proper hash and splits.
4. Try to submit with your network throttled/offline to verify the new error toast handlers.
5. Run unit tests (vitest run tests/unit/send-handler.test.ts) to verify coverage.