feat: durable transaction-tracking layer (closes #52)

[aojomo]

Summary

Implements a durable transaction-tracking layer for reliable on-chain resolution tracking after a learner signs and submits a transaction. Fixes the issue where loans.store.ts had no persistence, losing simulatedAmount, cached loans, and pending payments on restart.

Changes

Data Layer

• types/transaction.types.ts — Added pending transaction types
• services/transactions.service.ts — Added getTxStatus(txHash) for polling on-chain resolution

Persisted Queue

• src/transactions/pending-queue.ts — Full CRUD persisted queue using AsyncStorage
• Records: tx hash, type, target loan, installment index, amount, status, retry count

Background Poller

• src/transactions/transaction-poller.ts — Exponential backoff (2s→60s, 15 retries max)
• Per-tx concurrency locks to prevent race conditions
• On terminal status: idempotent loan state reconciliation

Loan Store Persistence

• stores/loans.store.ts — Zustand persist middleware with AsyncStorage
• markInstallmentPaid() with idempotency guard via processedTxHashes
• Bounded processed list (200 max), auto-transition to paid status
• clearLoans() resets all persisted state

Reconciliation

• app/_layout.tsx — Triggers on AppState→active via reconcilePendingTxs()
• Poller lifecycle starts when authenticated

UI Indicators

• app/(tabs)/index.tsx — Pending banner + recent activity section
• app/(tabs)/loans.tsx — Per-loan PendingTxBadge + full status sections

Tests

• src/transactions/tests/pending-queue.test.ts — 10 tests covering CRUD and edge cases
• stores/tests/loans.store.test.ts — 5 tests covering idempotency, auto-transition, persistence

Closes #46